Diagnostic Aids

WARNING: This page is about a different car, the 1995 Ford Aerostar. However, it is still accessible from the selected car via links, so may be relevant.

Perform this test when instructed during QUICK TEST or if directed by other test procedures.

Fuel-contaminated engine oil may affect Codes 136, 137, 139, 144, 171, 172, 173, 175, 176 and 177. If fuel-contaminated engine oil is suspected, remove PCV valve from valve cover and repeat QUICK TEST. If problem is corrected, change engine oil and filter.

Only use this test to diagnose:

HO2S and sensor connection.
Vacuum systems.
Fuel injector and/or fuel injector circuitry.
Powertrain Control Module (PCM).
Electrical circuits (HO2S, HO2S GND, INJ 1-8, VPWR and SIG RTN).

To prevent replacement of good components, be aware the following non-EEC areas may be cause of driveability concerns:

Ignition system.
Faulty evaporative emission system.
EGR and/or PCV system.
Air intake system.
Engine oil contamination.
Fuel system.
Intake manifold leaks.
Exhaust system leaks or restriction.
Electrical system.
Secondary air injection system.
Engine cooling system.

Fig 1: Identifying Fuel Injector & HO2S Connector Terminals (3.0L)

INDIVIDUAL INJECTOR RESISTANCE

Engine

⁽¹⁾ Ohms

All Models

11-18

⁽¹⁾	Resistance values are for a single injector.

MFI INJECTOR BANK RESISTANCE

Engine	Ohms
Aerostar	4.0-5.5

TEST PIN NO. 44 (HO2S-1) WIRE COLOR ID ⁽¹⁾

Application

Test Pin No.

Wire Color

Aerostar

Gray/Light Blue

⁽¹⁾	To determine if vehicle is equipped with MFI or SFI fuel system, see ENGINE/VIN ID article or appropriate WIRING DIAGRAM under WIRING DIAGRAMS .

TEST PIN NO. 43 (HO2S-2) WIRE COLOR ID ⁽¹⁾

Application

Test Pin No.

Wire Color

Aerostar

Red/Black

⁽¹⁾	To determine if vehicle is equipped with MFI or SFI fuel system, see ENGINE/VIN ID article or appropriate WIRING DIAGRAM under WIRING DIAGRAMS .

TEST PIN NO. 46 (HO2S-GROUND) WIRE COLOR ID ⁽¹⁾

Application

Test Pin No.

Wire Color

All Models

Gray/Red

⁽¹⁾	To determine if vehicle is equipped with MFI or SFI fuel system, see ENGINE/VIN ID article or appropriate WIRING DIAGRAM under WIRING DIAGRAMS .

FUEL INJECTOR TEST PIN NO. IDENTIFICATION ⁽¹⁾

Application

Test Pin No.

Wire Color

Cylinder No. 1

Tan

Cylinder No. 2

White

Cylinder No. 3

Brown/Yellow

Cylinder No. 4

Brown/Light Blue

Cylinder No. 5

Tan/Black

Cylinder No. 6

Light Green/Orange

⁽¹⁾	To determine if vehicle is equipped with MFI or SFI fuel system, see ENGINE/VIN ID article or appropriate WIRING DIAGRAM under WIRING DIAGRAMS .

TEST PIN NO. 29 (HO2S) WIRE COLOR ID ⁽¹⁾

Application

Test Pin No.

Wire Color

All Models

Gray/Light Blue

⁽¹⁾	To determine if vehicle is equipped with MFI or SFI fuel system, see ENGINE/VIN ID article or appropriate WIRING DIAGRAM under WIRING DIAGRAMS .

TEST PIN NO. 49 (HO2S-GROUND) WIRE COLOR ID (MFI MODELS) ⁽¹⁾

Application

Test Pin No.

Wire Color

All Models

Gray/Red

⁽¹⁾	To determine if vehicle is equipped with MFI or SFI fuel system, see ENGINE/VIN ID article or appropriate WIRING DIAGRAM under WIRING DIAGRAMS .

FUEL INJECTOR TEST PIN NO. IDENTIFICATION ⁽¹⁾

Application

Test Pin No.

Wire Color

Bank No. 1

Tan

Bank No. 2

Tan

⁽¹⁾	To determine if vehicle is equipped with MFI or SFI fuel system, see ENGINE/VIN ID article or appropriate WIRING DIAGRAM under WIRING DIAGRAMS .

1) Check For Contaminated Engine Oil Turn ignition off. Remove PCV valve from valve cover. Inspect PCV system for damage. Inspect valve for blockage and movement of valve plunger. Repair as necessary. Perform KOEO SELF-TEST and KOER SELF-TEST. Service ignition continuous memory code (if present) before KOER codes. If vehicle has a no-start condition, KOER Code 136, 137, 172, 173 or Continuous Memory Code 139, 144, 171, 175, 176, or 177 is set, install PCV valve and go to next step. If no codes are set, change engine oil and filter. Install PCV valve, and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

2) Check Fuel Pressure Turn ignition off. Release fuel pressure. Install fuel pressure gauge. Ensure manifold vacuum supply is connected to fuel pressure regulator (if equipped). Run engine at idle and check fuel pressure. If vehicle will not start, cycle ignition on and off several times. Check fuel pressure. If fuel pressure is within specification, go to next step. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If pressure is not within specification, go to FUEL SYSTEM in SYSTEM/COMPONENT TESTS - EEC-IV (3.0L) article in the ENGINE PERFORMANCE section.

3) Check System Ability To Hold Fuel Pressure Turn ignition on. Check fuel system for leaks. Repair or replace as necessary. If fuel system does not leak and pressure remains at specification for 60 seconds, go to next step for no-start condition, step 5) for MFI models, or step 6) for SFI models.

4) Fuel Delivery Test Release fuel system pressure. With fuel pressure gauge installed, pressurize fuel system as in step 2). Disconnect Inertia Fuel Shutoff (IFS) switch. See INERTIA FUEL SHUTOFF SWITCH LOCATION table. Crank engine for 5 seconds. If fuel pressure drop is greater than 5 psi after 5 seconds of cranking, EEC-IV system is not causing no-start condition. Check additional no-start tests in TESTS W/O CODES - EEC-IV article in the ENGINE PERFORMANCE section. If fuel pressure drop is less than 5 psi, remove fuel pressure gauge. Connect inertia switch. Go to step 7) for MFI or step 8) for SFI.

INERTIA FUEL SHUTOFF SWITCH LOCATION

Application	Location
Aerostar	Behind Right Kick Panel

5) Cylinder Balance Test (MFI) Connect tachometer to engine. Start and run engine at idle. Disconnect and reconnect injectors individually, noting RPM drop as each injector is disconnected. IAC motor will attempt to re-establish RPM. If each injector does not produce a momentary drop in RPM, go to step 7). If each injector produces a momentary drop in RPM, go to step 13) for Codes 136, 172 or 176, step 24) for Code 137, 173 or 177. Go to step 14) for all others.

CAUTION: If Continuous Memory Code 214 is present, repair as necessary before continuing with this test.

6) Cylinder Balance Test (SFI) Perform KOER SELF-TEST. After last code is displayed, wait 5-10 seconds. Enter CYLINDER BALANCE TEST by lightly goosing throttle (not WOT). Test time is about 2-3 minutes. To interpret codes retrieved from test, refer to CYLINDER BALANCE TEST SERVICE CODES table. If cylinder balance fault codes are present, go to step 8). If Code 136, 172 or 176 is present, go to step 13). If Code 137, 173 or 177 is present, go to step 24). If any other codes are present, go to step 14).

CYLINDER BALANCE TEST SERVICE CODES

Service Code

Application

Pass

Cylinder No. 1

Cylinder No. 2

Cylinder No. 3

Cylinder No. 4

Cylinder No. 5

Cylinder No. 6

Cylinder No. 7

Cylinder No. 8

538 ⁽¹⁾

Retest

⁽¹⁾	Code 538 indicates throttle was touched when test was ran and test is not completed. Code 538 could also be output if Continuous Memory Code 214 is present.

7) Check Injector & Harness Resistance (MFI) Turn ignition off. Disconnect PCM 60-pin connector. Inspect pins for damage. Install Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance of injector bank No. 1 between test pins No. 37 and 58 at breakout box. Measure resistance of injector bank No. 2 between test pins No. 37 and 59 at breakout box. Record resistances. See MFI INJECTOR BANK RESISTANCE table. If each resistance is within specification, go to step 12). If resistance is not as specified, repair open VPWR circuit for no-start condition, or go to step 9) for all other conditions.

8) Check Injector & Harness Resistance (SFI) Turn ignition off. Disconnect PCM 60-pin connector. Inspect connector for damaged pins, corrosion and loose wires. Repair as necessary. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure and record resistance between suspect injector circuit test pin and test pin No. 37. For no-start condition, measure and record resistance between any injector circuit test pin and test pin No. 37. Refer to INJECTOR BANK RESISTANCE table. If each resistance is within specification, go to step 12). If resistance is not as specified, repair open VPWR circuit for no-start condition, or go to next step for all other conditions.

9) Check Continuity Of Fuel Injector Harness Turn ignition off. Disconnect injector wiring harness connector at suspect injector. Measure resistance between test pins No. 37 and 57 at breakout box and VPWR terminal at suspect injector wiring harness connector. Also measure resistance between injector test pin(s) at breakout box and same injector circuit terminal at each injector wiring harness connector. If each reading is less than 5 ohms, go to next step. If any reading is 5 ohms or more, repair open circuit. Remove breakout box, reconnect all components and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

10) Check Injector Harness Circuit For Short To Power Or Ground Turn ignition off. With breakout box installed and PCM disconnected, disconnect suspect fuel injector wiring harness connector. Measure resistance between injector test pin(s) and test pins No. 37, 40, 46, 57 and 60. Measure resistance between injector test pin(s) at breakout box and chassis ground. If resistance is 10,000 ohms or more, replace injector per cylinder balance test service code and repeat QUICK TEST (SFI models) or go to next step (MFI models). If resistance is less than 10,000 ohms, repair short. Remove breakout box, reconnect all components and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

11) Isolate Faulty Injector Circuit Turn ignition off. Install breakout box, and disconnect PCM. Disconnect all injectors on suspect bank. With DVOM set on ohm scale, connect one injector and measure resistance between test pin No. 37 and test pin No. 58 (injector bank No. 1) or No. 59 (injector bank No. 2). Disconnect injector, and repeat process for all other injectors. See INDIVIDUAL INJECTOR RESISTANCE table. If all injectors are within specification, go to next step. If injectors are not as specified, replace injector. Remove breakout box, and reconnect components. Drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

12) Check Injector Drive Signal With ignition off and breakout box installed, connect PCM to breakout box. Connect a non-powered 12-volt test light as follows:

Connect test light between test pins No. 37 and 58 at breakout box. Connect test light between test pins No. 37 and 59 at breakout box.

Connect test light between test pin No. 37 and suspect injector test pin at breakout box. Crank or start engine. If test light glows dimly, system is operating correctly. Clean injectors, and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST and CYLINDER BALANCE TEST. If test light does not glow dimly (no light/bright light), replace PCM. Reconnect all components. Drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

13) Check Secondary Air Injection Operation If vehicle is equipped with pulse-air or does not have secondary air injection system, go to next step. A HO2S which is always lean could be caused by thermactor air being diverted upstream from HO2S. With dual HO2S, Code 172 applies to right or rear HO2S and Code 136 or 176 applies to left or front HO2S. Turn ignition off. Disconnect secondary air hose(s) from pump and vent to atmosphere. Perform KOER SELF-TEST. If Code 172, 136 or 176 is present, reconnect air hose(s) and go next step. If Continuous Memory Code 139, 144, 171, 176 or 177 is present, go to step 90). If Code 311, 312 or 313 is present, go to CIRCUIT TEST KC. If code(s) is not present, repair air injection system as necessary.

14) Check HO2S Integrity An HO2S which is always lean, slow to switch or does not switch could be caused by:

Moisture inside HO2S or connector causing a short to ground.
HO2S coated with contaminants.
HO2S circuit open.
HO2S circuit shorted to ground.

Turn ignition off. Inspect HO2S wiring harness for damage. Inspect HO2S and connector for signs of fluid entry. Repair or replace as necessary. Start engine, and run it at 2000 RPM for 2 minutes. Turn ignition off. Run KOER SELF-TEST. If fault codes are present, go to next step if equipped with MAP sensor or step 16) if equipped with MAF sensor. If no code is present, go to step 21).

15) Check HO2S (Engines With MAP Sensors) Vacuum or air leaks in non-EEC-IV areas could cause Code 172, 136 or 176. Check following possibilities before continuing:

Leaking vacuum actuated motor.
Leaking intake gasket.
EGR system.
PCV system.
Lead-contaminated HO2S.

Turn ignition off. Ensure MAP sensor output voltage is correct. See CIRCUIT TEST DF, step 3). Disconnect appropriate HO2S from wiring harness connector. Connect DVOM to HO2S signal terminal and HO2S GND or SIG RTN terminal at sensor wiring harness connector. Disconnect and plug vacuum line at MAP sensor. Start engine, and apply 10-14 in. Hg to MAP sensor. Run engine at 2000 RPM for 2 minutes. If voltage is 0.5 volt or more, go to step 17). If voltage is less than 0.5 volt, replace HO2S and repeat QUICK TEST.

16) Check HO2S (Engines With MAF Sensor) Purpose of this test is to verify HO2S can generate a voltage signal of greater than 0.5 volt during KOER SELF-TEST. Vacuum or air leaks in non-EEC-IV areas could cause Code 136, 172 or 176. Check following possibilities before continuing:

Leaking vacuum actuated motor.
Leaking intake gasket.
EGR system.
PCV system.
Unmetered air between MAF sensor and throttle body.
Lead-contaminated HO2S.

Turn ignition off. Disconnect appropriate HO2S from harness. Connect DVOM between HO2S signal terminal and HO2S GND or HO2S GND terminal at HO2S wiring harness connector. Run engine at 2000 RPM for 2 minutes. Repeat KOER SELF-TEST and monitor HO2S voltage. If voltage is 0.5 volt or more at end of self-test, go to next step. If voltage is less than 0.5 volt, replace HO2S and repeat QUICK TEST.

17) Check Continuity Of HO2S Ground Circuits Turn ignition off. Install breakout box, leaving PCM disconnected. Disconnect suspect HO2S wiring harness connector. Inspect connector for damage, moisture and corrosion. Repair as necessary. Measure resistance between HO2S test pin at breakout box and HO2S signal terminal at sensor wiring harness connector. Also measure resistance between test pin No. 46 (SFI) or No. 49 (MFI) at breakout box and HO2S GND terminal at HO2S wiring harness connector. If each resistance is less than 5 ohms, go to next step. If any resistance is 5 ohms or more, repair open circuit. Remove breakout box, and reconnect all components. Drive vehicle for 5 minutes at 55 MPH. Repeat QUICK TEST.

18) Check HO2S Circuit For Short To Ground Turn ignition off. Leave breakout box installed and PCM disconnected. Disconnect HO2S. Measure resistance between HO2S circuit test pin and test pins No. 40, 46 or 49 at breakout box. If any reading is less than 10,000 ohms, repair short to ground. Drive vehicle at 55 MPH for 5 minutes and repeat QUICK TEST. If all readings are 10,000 ohms or more, go to next step.

19) Check HO2S For Short To Ground Ensure ignition is off and PCM is disconnected. With HO2S disconnected, measure resistance between PWR GND and HO2S signal terminal at HO2S connector. Also measure resistance between HO2S signal terminal and HO2S GND terminal at HO2S wiring harness connector. If any reading is less than 10,000 ohms, replace HO2S. Drive vehicle for 5 miles at 55 MPH and repeat QUICK TEST. If both readings are 10,000 ohms or more, go to next step.

20) Check Resistance Of HO2S Heating Element Turn ignition off. Disconnect suspect HO2S. Inspect both ends of connector for damage, moisture and corrosion. Repair as necessary. Measure resistance between VPWR terminal and PWR GND circuit at HO2S connector. Hot to warm engine resistance should be 5-30 ohms. Cold engine temperature resistance is 2-5 ohms. If resistance is within specification, go to next step. If resistance is not as specified, replace HO2S and repeat QUICK TEST.

21) Check For Power At HO2S Harness Connector Disconnect HO2S. Turn ignition on. Measure voltage between negative battery terminal and VPWR terminal at HO2S wiring harness connector. If voltage is more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, repair open VPWR circuit. Reconnect HO2S and repeat QUICK TEST.

22) Check Continuity Of Power Ground Circuit Turn ignition off. Measure resistance between PWR GND circuit at HO2S wiring harness connector and negative battery terminal. If resistance is less than 5 ohms, perform following procedure as applicable.

For Codes, 139, 144, 171, 174, 175 and 178, go to step 90).
For all other codes, go to next step if equipped with MAP sensor, or replace PCM if equipped with MAF sensor. Drive vehicle for 5 miles at 55 MPH, and repeat QUICK TEST.

23) Attempt To Eliminate Code 136, 172 Or 176 (Engines With MAP Sensor) Turn ignition off. Connect PCM to breakout box. Connect HO2S. Disconnect and plug MAP sensor vacuum hose. Start engine and apply 10-14 in. Hg to MAP sensor. Increase engine speed to 2000 RPM for 2 minutes. Allow engine to return to idle. Perform KOER SELF-TEST. If diagnosing continuous memory codes, drive vehicle 5 miles at 55 MPH. If Code 136, 172 or 176 is still present, remove breakout box. Connect all components. If engine does not run rough, replace PCM, drive vehicle 5 miles at 55 MPH, and repeat QUICK TEST. If engine runs rough, go to CIRCUIT TEST S, step 2). If code is not present, remove breakout box and connect all components. HO2S input circuit and fuel delivery are okay; fault is in area common to all cylinders. See TESTS W/O CODES - EEC-IV article in the ENGINE PERFORMANCE section.

24) Diagnostic Test Code 137, 173 Or 177: Check HO2S SIGNAL For Short To Power On dual HO2S systems, Code 173 applies to right or rear sensor. Codes 137 and 177 apply to left or front sensor. HO2S continuously rich could be caused by:

Moisture inside wiring harness causing short to power.
HO2S shorted to power.

Turn ignition off. Disconnect suspect HO2S. Inspect connector for damage and repair as necessary. Turn ignition on. Measure voltage between HO2S and PWR GND terminals at HO2S wiring harness connector. If voltage is less than 0.5 volt, go to step 26). If voltage is 0.5 volt or more, go to next step.

25) Check For Short To Power Turn ignition off. Disconnect suspect HO2S. Inspect wiring harness for damage and repair as necessary. Disconnect PCM 60-pin connector. Install Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance between VPWR test pin No. 37/57 and HO2S test pin at breakout box. If resistance is 10,000 ohms or more, replace PCM. Remove breakout box, and reconnect all components. Drive vehicle 5 miles at 55 MPH. Repeat QUICK TEST. If resistance is less than 10,000 ohms, repair short circuit. Remove breakout box, and reconnect all components. Drive vehicle 5 miles at 55 MPH. Repeat QUICK TEST.

26) Check HO2S For Short To VPWR Circuit Turn ignition off. Disconnect HO2S. Measure resistance between VPWR terminal and HO2S terminal at HO2S connector. If resistance is less than 10,000 ohms, replace HO2S. Drive vehicle for 5 miles at 55 MPH, and repeat QUICK TEST. If resistance is 10,000 ohms or more, perform following procedure as applicable.

For Codes 137, 173 or 177, go to next step.
For Codes 171, 174, 175 and 178, or if vehicle is equipped with MAP sensor, go to step 28).
If vehicle is equipped with MAF sensor, go to step 30).

27) Attempt To Generate Code 136, 172 Or 176 Turn ignition off. Disconnect HO2S connector. Connect jumper wire between negative battery terminal and HO2S terminal at HO2S wiring harness connector. Repeat KOER SELF-TEST. If Code 136, 172 or 176 is present, remove jumper wire and go to next step if equipped with MAP sensor or step 30) if equipped with MAF sensor. If Code 136, 172 or 176 is not present, remove jumper wire. Disconnect PCM connector, and inspect it for damage. If connector is okay, replace PCM. Drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

Because MAP sensor greatly influences fuel control, MAP sensor may malfunction without setting Code 126. Next 2 steps test MAP vacuum circuit.

28) Check MAP Sensor For Vacuum Leaks Turn ignition off. Disconnect vacuum hose from MAP sensor. Inspect hose for damage and repair as necessary. Connect vacuum pump to sensor. Apply 18 in. Hg to MAP sensor. If sensor does not hold vacuum, replace MAP sensor. Drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST. If sensor holds vacuum, release vacuum and go to next step.

29) Check For Loss Of Vacuum To MAP Sensor Using vacuum "T", connect vacuum gauge in MAP sensor vacuum circuit. Start engine. Note vacuum reading after idle stabilizes. Turn ignition off. Remove "T" and vacuum gauge. Reconnect hose to MAP sensor. Connect vacuum gauge at a different manifold vacuum location. Start engine, and note vacuum reading. If readings do not differ by more than one in. Hg, go to next step. If readings differ by more than 1 in. Hg, inspect vacuum hoses for leaks, kinks and blockage. Repair as necessary. Connect all components, and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

30) HO2S Check Turn ignition off. Disconnect HO2S. Connect DVOM to HO2S terminal and HO2S GND terminal at HO2S connector. Create a vacuum leak to force HO2S to go lean. Remove PCV valve hose. Start engine. Raise engine speed to about 2000 RPM. DVOM should indicate less than 0.4 volt within 30 seconds. If voltage is not as specified, replace HO2S. Reconnect vacuum hoses, and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST. If voltage is as specified, go to step 90).

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 30) to step 90). No test procedures have been omitted.

90) Check Continuous Monitor Mode Start engine, and warm it to normal operating temperature. Raise engine speed to 2000 RPM for 2 minutes and then return to idle. Enter KOER wiggle test. See CONTINUOUS MONITOR MODE (WIGGLE TEST) under QUICK TEST. While observing DVOM or scan tester, shake or bend small sections of wiring harness from HO2S to PCM. Wiggle, shake or bend small sections of wiring harness from HO2S GND to PCM. If no fault is indicated, remain in CONTINUOUS MONITOR MODE and go to next step. If fault is indicated, isolate and repair fault as necessary. Clear codes. Repeat QUICK TEST.

91) Continuous Monitor Test Drive Check While still in KOER wiggle test, test drive vehicle for 5 miles at 55 MPH over smooth roads. Drive an additional 5 miles at 55 MPH over rough roads. If possible, drive vehicle through a pool of water to wet HO2S and connector. If fault is indicated, isolate fault and repair as necessary. Clear codes, and remove breakout box. Repeat QUICK TEST. If no codes are indicated, exit KOER wiggle test and go to next step.

92) Check HO2S Switching Turn ignition off. Inspect wiring harness for proper routing and insulation. Check for burnt, chaffed or open wires. Repair as necessary. Disconnect PCM 60-pin connector. Inspect connector for damaged pins, corrosion and loose wires. Repair as necessary. Install breakout box. Connect PCM to breakout box. Connect analog voltmeter to suspect HO2S test pin and HO2S GND at breakout box. Test drive vehicle for 5 miles at 55 MPH while observing voltmeter. HO2S should switch from .3 volt to .9 volt within 3 seconds. If HO2S does not switch as described, replace HO2S. Connect all components, and repeat QUICK TEST. If HO2S switches as described, fault cannot be identified or duplicated at this time. See INTERMITTENTS in the appropriate TESTS W/O CODES - EEC-IV article in the ENGINE PERFORMANCE section.