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 directed by QUICK TEST. This test is intended to diagnose:

MAF sensor.
Wiring harness circuits (VPWR, PWR GND, MAF and MAF RTN).
Powertrain Control Module (PCM).

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

Air cleaner element.
Inlet air duct.
Throttle body.

Code 159, retrieved during KOEO SELF-TEST, indicates voltage exceeded .7-volt test range. Code 159, retrieved during KOER SELF-TEST, indicates voltage is not within .2-1.5 volts operating range.

MAF SIGNAL VOLTAGE ⁽¹⁾

Application

Volts

Idle

20 MPH

1.1

40 MPH

1.7

60 MPH

2.1

⁽¹⁾	MAF signal voltage is typical for engines at normal operating temperature. Voltage signal may vary due to engine load and temperature.

NOTE: Code 159 may be caused by use of a garage exhaust ventilation system. Ensure vehicle is vented to outside atmosphere before repeating QUICK TEST.

1) Code 159: Check VPWR Circuit Voltage Code 159 indicates that MAF sensor was out of range during KOEO or KOER SELF-TEST. Possible causes for this fault are:

Air leak before or after MAF sensor.
Faulty MAF sensor.
Faulty IAC valve.
Open or grounded wiring harness.
Faulty PCM.

Repeat QUICK TEST. If Code 412 is present, go to CIRCUITTEST KE, step 1). If Code 411 is present, go to TEST KE, step 15). If Code 411 or 412 is not present, go to next step.

2) Check VPWR Circuit Voltage Turn ignition off. Disconnect MAF sensor. Set DVOM on 20-volt scale. Turn ignition on. Measure voltage between VPWR terminal at MAF sensor wiring harness connector and negative battery terminal. If voltage is less than 10.5 volts, repair open in VPWR circuit and repeat QUICK TEST. If voltage is 10.5 volts or more, go to next step.

3) Check MAF Sensor Ground Turn ignition on. Disconnect MAF sensor. Measure voltage between VPWR terminal and PWR GND terminal at MAF sensor connector. If voltage is less than 10.5 volts, repair open PWR GND circuit. Reconnect MAF sensor and repeat QUICK TEST. If voltage is 10.5 volts or more, go to step 13).

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 3) to step 10). No test procedures have been omitted.

10) Continuous Memory Code 157: Check For Intermittent Sensor Continuous Memory Code 157 indicates MAF signal was less than 0.4 volt sometime during last 80 warm-up cycles. Possible causes for this fault are:

Open MAF circuit.
MAF circuit shorted to ground.
Faulty MAF sensor.
MAF sensor disconnected.

Clear continuous memory. Start engine and allow to idle for 5-10 minutes. Increase engine speed to 1500 RPM for 5 seconds. Perform KOEO SELF-TEST. If Code 157 is present, go to step 13). If Code 157 is not present, go to next step.

11) Monitor MAF Circuit Under Simulated Road Shock Turn ignition off. 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 connected. Connect DVOM between test pin No. 9 and test pin No. 50 at breakout box. DVOM should read about 0.4 volt. Start engine and allow to idle. Lightly tap on MAF sensor and wiggle wiring harness to simulate road conditions. If sudden change in DVOM reading occurs, fault is indicated. If fault is indicated, replace MAF sensor and repeat QUICK TEST. If no fault is indicated, leave DVOM connected and go to next step.

12) Check Wiring Harness For Open Or Short Circuit Turn ignition on. Shake and bend small sections of wiring harness starting at MAF sensor and working toward dash panel. Shake and bend small sections of wiring harness starting at dash panel and working toward PCM. If fault is indicated, isolate and repair as necessary. If fault is not indicated, problem is intermittent and cannot be duplicated at this time.

13) Check For Air Leak At MAF Sensor Check for broken or loose air outlet tube clamps at throttle body and air cleaner assembly. Repair or replace as necessary. If no faults are found, go to next step.

14) Check MAF & VPWR Circuit Continuity Turn ignition off. Disconnect MAF sensor. Disconnect PCM. Install Breakout Box (49-UN01-058), leaving PCM disconnected. Measure resistance between VPWR terminal at MAF sensor wiring harness connector and test pins No. 37 and 57 at breakout box. Also, measure resistance between MAF terminal at MAF sensor wiring harness connector and test pin No. 50 at breakout box. If resistance is more than 5 ohms, repair open circuit. Remove breakout box, reconnect all components and repeat QUICK TEST. If resistance is 5 ohms or more, go to next step.

15) Check MAF Signal For Short To Ground Turn ignition off. Disconnect MAF sensor. Discionnect PCM. Measure resistance between test pin No. 50 and test pins No. 9, 40 and 60 at breakout box. If any reading is less than 10,000 ohms, repair short circuit. Remove breakout box, reconnect all components and repeat QUICK TEST. If all readings are 10,000 ohms or more, go to next step.

16) Check Continuity of PWR GND Circuit Turn ignition off. Disconnect MAF sensor. Disconnect PCM. Measure resistance between PWR GND terminal at MAF sensor wiring harness connector and negative battery terminal. If resistance is 10,000 ohms or more, repair open PWR GND circuit. Remove breakout box, reconnect all components and repeat QUICK TEST. If resistance is less than 10,000 ohms, go to next next step.

17) Check MAF RTN Circuit Continuity Turn ignition off. Measure resistance between MAF RTN terminal at MAF sensor wiring harness connector and test pin No. 9 at breakout box. If resistance is more than 5 ohms, repair open circuit. Remove breakout box, reconnect all components and repeat QUICK TEST. If resistance is 5 ohms or more, go to next step.

18) Check MAF Circuit For Short To Ground Turn ignition off. Disconnect MAF sensor. Connect PCM to breakout box. Measure resistance between test pin No. 50 and test pins No. 9, 40 and 60 at breakout box. If any reading is less than 10,000 ohms, replace PCM. Remove breakout box, reconnect all components and repeat QUICK TEST. If all readings are 10,000 ohms or more, go to next step.

19) Check MAF Circuit Output Turn ignition off. Reconnect MAF sensor. Start engine. Measure resistance between test pin No. 50 at breakout box and negative battery terminal. If voltage is not 0.6-0.7 volts, replace MAF sensor. Remove breakout box, reconnect all components and repeat QUICK TEST. If voltage is 0.6-0.7 volts, go to next step.

20) Check MAF Circuit Output With engine running, measure resistance between test pin No. 9 and test pin No. 50 at breakout box. If voltage is .36-1.50 volts, replace PCM. Remove breakout box, reconnect all components and repeat QUICK TEST. If voltage is not .36-1.50 volts, replace MAF sensor. Remove breakout box, reconnect all components and repeat QUICK TEST.

NOTE: Code 158 may be caused by foreign material blocking MAF sensor screen creating an airflow restriction. If contaminates are found on screen, check air filter installation in air cleaner tray and for proper sealing of air cleaner/tube before proceeding.

21) Code 158: Repeat KOEO SELF-TEST Turn ignition off. Disconnect MAF sensor. Start engine and allow it to idle for one minute. Turn ignition off. Repeat KOEO SELF-TEST. If Code 157 is present, replace MAF sensor and repeat QUICK TEST. If Code 157 is not present, go to next step.

22) Check MAF Circuit For Short To VPWR Turn ignition off. Disconnect PCM 60-pin connector and inspect for damaged pins, corrosion and loose wires. Repair as necessary. Measure resistance between MAF and VPWR terminals at MAF sensor wiring harness connector. If resistance is 10,000 ohms or more, replace PCM. Remove breakout box, reconnect all components and repeat QUICK TEST. If resistance is less than 10,000 ohms, repair short circuit. Remove breakout box, reconnect all components and repeat QUICK TEST. Remove breakout box, reconnect all components and repeat QUICK TEST.