So I've seen quite a few threads on MAF Sensor faults and failures and I came across some information I thought would be worthwhile to share in order to clear the issue(s). I hope this helps someone in the future.


The Vehicle - 1994 Ford Ranger Splash with V6 engine and 5 speed manual transmission. (OHV and Vulcan)

The Symptom - When engine is started, it runs really rough and is very hard to keep running. Ran a code check on it due to CEL on/off but now is on all the time. Codes were 157 and 158 which referred to the MAF sensor faults, low/high voltage (respectively). Sensor replaced, but nothing changed.

Owner Comment - "The puzzling part about it is if you can keep it running long enough to unplug the MAF sensor, then it runs like a charm with it unplugged! Light stays on though."

The obvious is with the sensor disconnected there is no fault but poor gas mileage and performance will persist and because it is disconnected also the CEL will continue to stay on fulltime.


First - Make sure the air intake tube is on tight and there are no leaks in it. Check the air filter and housing and check the Maf sensor screen for debris or obstruction.

If all checks out properly you may have to proceed tot he next step.

Step II - Pinpoint test for these 2 codes. Make sure you follow them very closely.

DC10 CONTINUOUS MEMORY DIAGNOSTIC TROUBLE CODE (DTC) 157: CHECK FOR INTERMITTENT SENSOR

* Poor continuity in MAF sensor harness or connectors.
* Intermittent open or short in MAF sensor or harness.
* Damaged MAF sensor.
* Idle Air Control (IAC) system failure (at closed throttle position).

Continuous Memory DTC 157 indicates the Mass Air Flow (MAF) sensor signal went below 0.4 volt sometime during the last 80 warm-up cycles.

Possible causes:
- Clear Continuous Memory (refer to Quick Test Appendix, Section 5A ).
- Start engine and idle for 5 to 10 minutes.
- Vehicle in PARK or NEUTRAL.
- Suddenly goose (greater than a half second) the throttle, approaching 1500 rpm.
- Allow to return to idle.
- Then, goose the throttle again, approaching 1500 rpm.
- Key on, engine off.
- Run Self-Test to observe Continuous Memory DTCs.

Is Continuous Memory DTC 157 present?

Yes
GO to DC13

No
GO to DC11 .


DC11 MONITOR MAF CIRCUIT UNDER SIMULATED ROAD SHOCK

- Key off.
- Disconnect Powertrain Control Module (PCM). Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary.
- Install breakout box, reconnect PCM.
- Connect DVOM between Test Pin 50 (SFI vehicles) or Test Pin 14 (MFI vehicles) and Test Pin 9 (SFI vehicles) or Test Pin 15 (MFI vehicles) at the breakout box.
- Key on, engine running.
- Lightly tap on MAF sensor and wiggle harness connector to simulate road shock.

Note: MAF voltage is normally above 0.4 volt. A sudden change down from this minimum limit indicates a fault.

Is a fault indicated?

Yes
DISCONNECT and INSPECT MAF sensor connector. If OK, REPLACE MAF sensor. CLEAR Continuous Memory (REFER to Quick Test Appendix, Section 5A ). RERUN Quick Test.

No
LEAVE DVOM and PCM connected. GO to DC12 .


DC12 CHECK VEHICLE HARNESS FOR INTERMITTENT OPENS OR SHORTS

- Key on, engine off.
- DVOM connected between Test Pin 50 (SFI vehicles) or Test Pin 14 (MFI vehicles) and Test Pin 9 (SFI vehicles) or Test Pin 15 (MFI vehicles) at the breakout box.
- Grasp the vehicle harness closest to the MAF sensor connector. Shake and bend a small section of the vehicle harness while working toward the dash panel. Also wiggle, shake and bend the vehicle harness from the dash panel to the PCM.

Is a fault indicated?

Yes
ISOLATE fault and SERVICE as necessary. CLEAR Continuous Memory (REFER to Quick Test Appendix, Section 5A ). RERUN Quick Test.

No
RECONNECT all components. Unable to duplicate and/or identify fault at this time. For further diagnosis, REFER to Section 7A , Intermittent Fault Diagnosis.


DC13 DIAGNOSTIC TROUBLE CODE (DTC) 157 OR 129: AIR LEAK AT MAF SENSOR

- Open MAF circuit.
- Open VPWR circuit to MAF sensor.
- Open PWR GND circuit to MAF sensor.
- Open MAF RTN circuit to MAF sensor.
- MAF circuit shorted to ground.
- Damaged Powertrain Control Module (PCM).
- Damaged MAF sensor.
- Air leak before or after MAF sensor.
- MAF sensor disconnected.
- Idle Air Control (IAC) system failure (at closed throttle position).


Engine Running DTC 129 indicates insufficient MAF change during Dynamic Response Test.

Possible causes:
* Check for broken/loose air outlet tube clamps (throttle body and air cleaner assembly ends), cracks/holes in air outlet tube, worn gaskets between MAF sensor and air cleaner assembly (including intermediate tube on 3.8L SFI Taurus/Sable and Continental).

Is a fault indicated?

Yes
SERVICE as necessary. RECONNECT all components. RERUN Quick Test.

No
GO to DC14 .


DC14 CHECK CONTINUITY OF MAF AND VPWR CIRCUITS

- Key off.
- Disconnect MAF sensor.
- Disconnect Powertrain Control Module (PCM). Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary.
- Install breakout box, leave PCM disconnected.
- Measure resistance between VPWR circuit at the MAF sensor vehicle harness connector and Test Pins 37 and 57 at the breakout box.
- For MFI vehicles:
- Measure resistance between MAF circuit at the MAF sensor vehicle harness connector and Test Pin 14 at the breakout box.
- For SFI vehicles:
- Measure resistance between MAF circuit at the MAF sensor vehicle harness connector and Test Pin 50 at the breakout box.

Is each resistance less than 5.0 ohms?

Yes
GO to DC15 .

No
SERVICE open circuit. REMOVE breakout box. RECONNECT all components. RERUN Quick Test.


DC15 CHECK MAF CIRCUIT FOR SHORTS TO GROUND AND MAF RTN CIRCUIT

- Key off.
- MAF sensor and PCM disconnected.
- Measure resistance between Pin 50 and Pins 9, 40 and 60 at the PCM connector.

Is each resistance greater than 10,000 ohms?

Yes
GO to DC16 .

No
SERVICE short circuit(s). REMOVE breakout box. RECONNECT all components. RERUN Quick Test.


DC16 CHECK PWR GND CIRCUIT CONTINUITY

- Key off.
- MAF sensor disconnected.
- Breakout box installed, PCM disconnected.
- Measure resistance between PWR GND circuit at the MAF sensor vehicle harness connector and battery negative post.

Is resistance less than 10 ohms?

Yes
GO to DC17 .

No
SERVICE open circuit. REMOVE breakout box. RECONNECT all components. RERUN Quick Test.


DC17 CHECK MAF RTN CIRCUIT CONTINUITY

- Key off.
- MAF sensor disconnected.
- PCM disconnected.
- Measure resistance between MAF RTN circuit at the MAF sensor vehicle harness connector and Test Pin 9 at the breakout box.

Is resistance less than 5.0 ohms?

Yes
GO to DC18 .

No
SERVICE open circuit. REMOVE breakout box. RECONNECT all components. RERUN Quick Test.


DC18 CHECK MAF CIRCUIT FOR SHORT TO GROUND

- Key off.
- MAF sensor disconnected.
- Breakout box installed.
- Connect PCM to breakout box.
- Disconnect scan tool from data link connector (if applicable).
- Measure resistance between Test Pin 50 and Test Pins 9, 40 and 60 at the breakout box.

Is each resistance greater than 10,000 ohms?

Yes
RECONNECT scan tool. GO to DC19 .

No
REPLACE PCM. REMOVE breakout box. RECONNECT MAF sensor. RERUN Quick Test.


DC19 CHECK MAF CIRCUIT OUTPUT

- Key off.
- Reconnect MAF sensor.
- Breakout box installed, PCM connected.
- Key on, engine running.
- Measure voltage between Test Pin 50 at the breakout box and negative battery post.

Is voltage between .36 and 1.50 volts?

Yes
GO to DC20 .

No
REPLACE MAF sensor assembly. REMOVE breakout box. RECONNECT PCM. RERUN Quick Test.


DC20 CHECK MAF CIRCUIT OUTPUT

- Key off.
- Reconnect MAF sensor.
- Breakout box installed, PCM connected. (If you don't have a breakout box, back probe the light blue/red and tan/light blue wires at the MAF conne
- Key on, engine running.
- Measure voltage between Test Pin 50 and Test Pin 9 at the breakout box.

Is voltage between 0.36 and 1.50 volts?

Yes
REPLACE PCM. REMOVE breakout box. RERUN Quick Test.

No
REPLACE MAF sensor. REMOVE breakout box. RECONNECT PCM. RERUN Quick Test.

----------

DC21 DIAGNOSTIC TROUBLE CODE (DTC) 158:
RERUN SELF-TEST WITH MAF SENSOR DISCONNECTED

DTC 158 indicates the Mass Air Flow (MAF) sensor signal went above 4.5 volts during normal engine operation (continuous) or during Self-Test.

Note: DTC 158 could be generated by foreign material blocking the Mass Air Flow sensor screen creating an air flow restriction. If contaminants are found on the screen, check air filter installation in air cleaner tray and proper sealing of air cleaner/tube before proceeding.
- Key off.
- Disconnect MAF sensor.
- Start engine, idle one minute.
- Key off.
- Run Key On Engine Off Self-Test.

Is DTC 157 present?

Yes
REPLACE MAF sensor. RERUN Quick Test.

No
GO to DC22 .


DC22 CHECK MAF CIRCUIT FOR SHORT TO VPWR

- Key off.
- MAF sensor disconnected.
- Disconnect Powertrain Control Module (PCM). Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary.
- Disconnect scan tool from data link connector (if applicable).
- Measure resistance between MAF circuit and VPWR circuit at the MAF sensor vehicle harness connector.

Is resistance greater than 10,000 ohms?

Yes
REPLACE PCM. REMOVE breakout box. RECONNECT MAF sensor. RERUN Quick Test.

No
SERVICE short circuit. REMOVE breakout box. RECONNECT all components. RERUN Quick Test.

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