ftltmp

I had a friend of mine call me tonight, and asked me about his 08. The check engine light come on and the code its throwing is a mass air flow sensor. He changed it reset the light and it comes back on. Anyone know what may be causing this?

xcc_rider

Post the actual code (there's several with different diagnostic checks) and you'll get more help.

ftltmp

P0101 maf malfunction

xcc_rider

Theory of Operation
The Mass Air Flow Sensor is a frequency based device. A constant voltage is applied to a heated wire. This wire is positioned in the air stream and is heated by the electr!cal current that the voltage produces. As air flows across it, it cools down. The heated wire or film is a positive temperature coefficient (ptc) resistor. This means that it's resistance drops when it's temperature drops. The drop in resistance allows more current to flow through it in order to maintain the programmed temperature. This current is changed to a frequency which is sent to the ECM and interpreted as air flow. Adjustments for air temperature and humidity are taken into consideration since they also affect the temperature of the heated wire or film.
This diagnostic has two parts, a key on monitor and a monitor that runs in operating conditions when the EGR valve is closed.
The key on monitor diagnostic fails if the Mass Air Flow Sensor reads a non-zero value at key on, engine off.
The engine running monitor compares an estimated charge air flow value to the Mass Air Flow sensor reading. This monitor only runs when the EGR valve is commanded closed. During this time the MAF sensor value should equal the charge air flow estimate. The ECM will light the MIL lamp if this diagnostic runs and fails in two consecutive drive cycles. The ECM will turn off the MIL lamp when the diagnostic runs and passes in four consecutive drive cycles .
• When Monitored: Ignition on .
• Set Condition: Flow detected is greater than allowable threshold at key on/engine off.
While running and EGR flow equals zero the Mass Air Flow sensor value does not equal the charge air flow estimate.
Possible Causes
TURBOCHARGER, EGR OR INTAKE AIR SYSTEM RELATED DTC'S
MAF SENSOR
AIR FILTER
AIR INTAKE LEAK
ENGINE HARNESS OR CONNECTORS

Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE DIAGNOSIS AND TESTING) :
Diagnostic Test 1. OTHER DTC'S
Ignition on, engine not running. With the scan tool, read DTC's. Are any other DTC'S present?
Yes »Repair all other DTC's first. Perform POWERTRAIN VERIFICATION TEST - B.7L. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No »Go To 2
2 . MAF SENSOR With the scan tool in Sensors, read the Mass Air Flow sensor. Does the Mass Air Flow sensor read 0 +/- 2Ib/min?
Yes »Go To 3 No
»Replace MAF sensor. Perform POWERTRAIN VERIFICATION TEST - 6.7L (Refer to 9 - ENGINE - STANDARD PROCEDURE)
3. REPLACE AIR FILTER Remove lid from air fitter box. Remove air filter and inspect for excessive debris or filter media damage. Ensure the filter was installed properly. Was the air filter damaged or full of debris?
Yes »Replace air filter. Perform POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 9 .. ENGINE .. STANDARD PROCEDURE)
No »Go To 4
4. MAF SENSOR DIRTY Turn the Ignition off. Remove MAF sensor from air box. Is it free from debris?
Yes »Go To 5 No
»Clean sensor using low pressure air and reinstall, repair complete. Perform POWERTRAIN VERIFICATION TEST ,- 6.7L. (Refer to 9 - ENGINE .. STANDARD PROCEDURE)
5. AIR LEAK IN INTAKE SYSTEM Check for an air intake leak. Any leak between the air box to the intake manifold could cause this fault. Were there any air intake leaks?
Yes »Replace air filter. Perform POWERTRAIN VERIFICATION TEST .. 6.7L. (Refer to 9 - ENGINE .. STANDARD PROCEDURE)
No
»Refer to the INTERMITTENT CONDITION - DIESEL Symptom. (Refer to 9 .. ENGINE .. DIAGNOSIS AND TESTING)

In the diagnostics for the map and exhaust pressure sensors it states that the three readings are compared to the expected values and the erroneous reading can trigger a code. Sometimes that code isn't for the correct sensor so if the above doesn't solve the problem then I would start with cleaning the other two sensors (especially the map if he's running a reusable aftermarket air filter).

And only to rule out all possibilities, there are two sensors at the filter so is he sure he replaced the correct sensor?