Diagnostic Aids

WARNING: This page is about a different car, the 1995 Mazda MX-6 and 1995 Mazda 626. 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 172 and 173. 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. This test is intended to diagnose:

HO2S and sensor connection.
Vacuum systems.
Fuel injector and/or fuel injector circuitry.
Powertrain Control Module (PCM).
Wiring harness circuits (HO2S, HO2S GND, INJ 1-4, 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 leaks.
Exhaust system leaks or restriction.
Electrical system.
Engine cooling system.

1) Codes 172 & 173: 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 and KOER SELF-TEST. Service ignition continuous memory code (if present) before servicing KOER codes. If vehicle is a no-start, 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. If Code 172 or 173 is set, go to next step.

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. See FUEL PRESSURE SPECIFICATIONS table. If fuel pressure is within specification, go to next step. If pressure is not within specification, check and repair fuel system as necessary. See FUEL SYSTEM in appropriate TESTS W/CODES - 2.0L M/T article.

FUEL PRESSURE SPECIFICATIONS

Engine	KOER Pressure psi (kg/cm² )	KOEO Pressure psi (kg/cm² )
2.0L A/T	30-38 (2.1-2.7)	37-46 (2.6-3.3)

3) Check System Ability To Hold Fuel Pressure Turn ignition on. Check fuel system for leaks. Repair or replace as necessary. Pressurize fuel system. See FUEL PRESSURE SPECIFICATIONS table. If fuel system does not leak and pressure remains at specification for 5 minutes, go to next step for no-starts, or step 6). If pressure is not within specification,specification, go to next step. If pressure is not within specification, check and repair fuel system as necessary. See FUEL SYSTEM in TESTS W/CODES - 2.0L M/T article.

4) Fuel Delivery Test Turn ignition off. With fuel gauge installed, install fuel pressure gauge between fuel filter and main fuel hose. Connect a jumper wire between fuel pump terminal of Data Link Connector (DLC) and ground. Turn ignition on for 10 seconds to operate fuel pump. Turn ignition off and remove jumper wire. Observe fuel pressure for 5 minutes. If fuel pressure is greater than 21 psi, EEC-IV system is not causing no-start condition. Remove fuel pressure gauge and check additional no-start tests in TESTS W/O CODES article. If fuel pressure is less than 21 psi, remove fuel pressure gauge and go to step 8).

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 4) to step 6). No test procedures have been omitted.

6) Cylinder Balance Test 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. If cylinder balance test service codes are present, go to step 8). See CYLINDER BALANCE TEST SERVICE CODES table to interpret codes. If KOER Code 244 (CID) is output, cylinder balance test has been aborted. Go to TEST DR. Code 538 indicates throttle was touched when test was run and test is not completed. Code 538 could also be output if Continuous Memory Code 214 (CID) is present. If Continuous Memory Code 214 is present, go to TEST DR. For Code 173, go to step 24). For all other codes, go to step 14).

CYLINDER BALANCE TEST SERVICE CODES

Service Code	Application
90	Pass
10	Cylinder No. 1
20	Cylinder No. 2
30	Cylinder No. 3
40	Cylinder No. 4
538	Retest

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 6) to step 8). No test procedures have been omitted.

8) Check Injector & Harness Resistance Turn ignition off. Disconnect PCM 60-pin connector and inspect connector for damaged pins, corrosion and loose wires. Repair as necessary. Install Breakout Box (49-UN01-058), leaving PCM disconnected. Measure and record resistance between suspected injector circuit test pin and test pin No. 37 at breakout box. For no-start, measure and record resistance between any injector circuit test pin and test pin No. 37 at breakout box. See INDIVIDUAL INJECTOR RESISTANCE table. If each resistance is within specification, go to step 12). If resistance is not as specified, repair open circuit on VPWR circuit for no-start, or go to next step for all other conditions.

INDIVIDUAL INJECTOR RESISTANCE @ 68°F (20°C)

Engine	Ohms
2.0L A/T	12-16

9) Check Fuel Injector Harness Continuity Turn ignition off. Disconnect suspect fuel injector wiring harness connector. Set DVOM to 200-ohm scale. With breakout box installed and PCM disconnected, 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 each 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 at breakout box. Also, measure resistance between injector test pin(s) at breakout box and chassis ground. If each resistance is 10,000 ohms or more, replace injector per cylinder balance test service code and repeat QUICK TEST. If each resistance is less than 10,000 ohms, repair short circuit. Remove breakout box, reconnect all components and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 10) to step 12). No test procedures have been omitted.

12) Check Injector Drive Signal Turn ignition off. With breakout box installed, connect PCM to breakout box. Using a non-powered 12-volt test light, connect test light between test pin No. 37 and suspect fuel injector test pin at breakout box. Crank or start engine. If test light glows dimly, system is operating correctly. Remove breakout box, reconnect all components and specification, go to next step. If pressure is not within specification, check and repair fuel system as necessary. See FUEL SYSTEM in appropriate TESTS W/CODES - 2.0L M/T article. Also check ignition system for other possible problems. See IGNITION SYSTEM in appropriate SYSTEM/COMPONENT TESTS - 2.0L article. Drive vehicle for 5 miles at 55 MPH and repeat QUICK TEST and CYLINDER BALANCE TEST. If test light does not glow dimly (no light/bright light), replace PCM. Remove breakout box, reconnect all components and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 12) to step 14). No test procedures have been omitted.

14) Check HO2S Integrity - A 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 codes are present, go to step 16). If no codes are present, go to step 21).

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 14) to step 16). No test procedures have been omitted.

16) Check HO2S 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 172. 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. Connect DVOM between HO2S terminal and SIG RTN, 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 HO2S Ground Circuit Continuity Turn ignition off. Install breakout box, leaving PCM disconnected. Disconnect suspect HO2S. Inspect connector for damage, moisture and corrosion. Repair as necessary. Measure resistance between HO2S test pin at breakout box and HO2S terminal at HO2S wiring harness connector. Measure resistance between SIG RTN test pin at breakout box and SIG RTN terminal at HO2S wiring harness connector. Where applicable, measure resistance between HO2S GND and SIG RTN at breakout box. 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 miles at 55 MPH and repeat QUICK TEST.

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

19) Check HO2S For Short To Ground Turn ignition off. Disconnect HO2S. With breakout box installed and PCM disconnected, measure resistance between PWR GND and HO2S terminal at HO2S wiring harness connector. Also measure resistance between HO2S GND and/or SIG RTN terminal at HO2S wiring harness connector. If resistance is less than 10,000 ohms, replace HO2S. Remove breakout box and reconnect all components. Drive vehicle for 5 miles at 55 MPH and repeat QUICK TEST. If resistance is 10,000 ohms or more, replace PCM. Remove breakout box and reconnect all components. Drive vehicle for 5 miles at 55 MPH and repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 19) to step 21). No test procedures have been omitted.

21) 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 IGN RUN circuit and PWR GND circuit at HO2S wiring harness connector. Resistance at 68°F (20°C) should be 6 ohms. If resistance is as specified, go to next step. If resistance is not as specified, replace HO2S and repeat QUICK TEST.

22) Check For Power At HO2S Harness Connector Disconnect HO2S. Turn ignition on. Measure voltage between IGN RUN and PWR GND circuits at HO2S wiring harness connector. If voltage is 10.5 volts or more, HO2S system and fuel delivery are okay. HO2S may have cooled before KOER SELF-TEST. If driveability symptom continues, fault is in area common to all cylinders (vacuum leak, fuel contamination, EGR, ignition system, etc.). See TESTS W/O CODES article. If voltage is less than 10.5 volts, go to next step.

23) Check Continuity Of Power Ground Circuit Turn ignition off. Disconnect HO2S. Measure resistance between PWR GND circuit at HO2S wiring harness connector and negative battery terminal. If resistance is less than 5 ohms, repair open in IGN RUN circuit, reconnect HO2S and repeat QUICK TEST. If resistance is 5 ohms or more, repair open in PWR GND circuit, reconnect HO2S and repeat QUICK TEST.

24) Code 173: Check HO2S Signal For Short To Power 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, moisture, corrosion and loose wires. Repair as necessary. Turn ignition on. Measure voltage between HO2S and PWR GND terminals at HO2S wiring harness connector. If reading is less than 0.5 volt, go to step 26). If reading is 0.5 volt or more, go to next step.

25) Check For Short To Power Turn ignition off. Inspect HO2S wiring harness for damage and repair as necessary. Disconnect PCM 60-pin connector and inspect for damaged pins, corrosion and loose wires. Repair as necessary. Install Breakout Box (49-UN01-058), leaving PCM disconnected. Disconnect suspect HO2S. Measure resistance between HO2S test pin and IGN/RUN test pin at breakout box. If reading 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 reading 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 IGN RUN Circuit Turn ignition off. Disconnect HO2S. Measure resistance between IGN RUN terminal and HO2S SIG terminal at HO2S wiring harness connector. If resistance is 10,000 ohms or more, go to next step. If resistance is less than 10,000 ohms, replace HO2S. Drive vehicle for 5 miles at 55 MPH and repeat QUICK TEST.

27) Attempt To Generate Code 172 Turn ignition off. Disconnect HO2S. Using jumper wire, connect HO2S terminal at HO2S wiring harness connector to negative battery terminal. Repeat KOER SELF-TEST. If Code 172 is present, remove jumper wire and go to step 30). If Code 172 is not present, remove jumper wire. Reconnect HO2S. Disconnect PCM connector and inspect for damage. If connector is okay, replace PCM. Drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 27) to step 30). No test procedures have been omitted.

30) HO2S Check Turn ignition off. Disconnect HO2S. Connect DVOM to HO2S terminal and HO2S GND or SIG RTN terminal at HO2S wiring harness 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

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. Remove breakout box, clear codes and 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. Remove breakout box, clear codes and repeat QUICK TEST. If no fault is indicated, exit KOER wiggle test and go to next step.

92) Check HO2S Switching Turn ignition off. Inspect EEC-IV wiring harness for proper routing and insulation. Check for burnt, chaffed or open wires. Repair as necessary. Disconnect PCM 60-pin connector and inspect for damaged pins, corrosion and loose wires. Repair as necessary. Warm engine to normal operating temperature, then run at idle. Disconnect HO2S. Install breakout box. Connect PCM to breakout box. Connect analog voltmeter to suspect HO2S test pin and HO2S GND at breakout box. Operate engine at 300 RPM until voltmeter indicates about .55 volts. Increase and decrease engine speed suddenly several times and observe voltmeter. Voltage should indicate 0.5-1.0 volt when speed is increased and 0-0.5 volt when speed is decreased. If HO2S does not switch as described, replace HO2S. Remove breakout box, reconnect all components and repeat QUICK TEST. If HO2S switches as described, fault cannot be identified or duplicated at this time. See INTERMITTENTS in TESTS W/O CODES article.