2002 Ford F150 P0401/P0402/P1131/P1151/P0171 Diagnosis: DPFE Sensor, O2 Sensors & Emission Test Failure Fix

DPFE sensors are a common issue on 2002 Ford F150 models with P0401 and P0402 codes. Ford has issued a recall for these vehicles—any time either P0401 or P0402 appears, the DPFE sensor should be replaced. What engine size do you have? The P1131 and P1151 codes are related to oxygen (O2) sensors. Do you have a K&N air filter installed? If so, it may have contaminated your Mass Air Flow (MAF) sensor, contributing to the O2 sensor faults. Try cleaning the MAF sensor. Also check for vacuum leaks—inspect all hoses and connections. Look for corrosion in O2 sensor wiring. Any exhaust leaks near or before an O2 sensor can trigger these codes. Check the PCV system for cracks or collapse. Ensure there are no air leaks between the MAF sensor and throttle body. Have you noticed any driveability issues?

4.2L engine. No K&N filter installed. No driveability problems reported. I recently cleaned the Mass Air Flow (MAF) sensor—located on the intake manifold, held by two bolts and requiring a 10mm socket to remove. The diaphragm inside showed a whitish separation; it was dirty but not clear if I cleaned the correct area. I used quick-evaporating solvent and reassembled it properly. I’ve purchased a DPFE sensor (left bank upstream O2 sensor) from AZ and will replace it. After clearing codes, I’ll take the vehicle back to the emissions testing facility. It’s likely to fail again due to drive cycle reset—but at least I can get the title tag with this intervention. Thank you for your response.

Did you clean the MAF or the MAP sensor?

Apologies—meant to say Mass Air Flow (MAF) sensor. It’s the one bolted to the intake line, secured by two bolts. A 10mm socket was needed to remove it. The diaphragm inside had a whitish separation; it appeared dirty but I’m unsure if I cleaned the correct section. I used quick-evaporating solvent and reinstalled it properly.

Today, I also inspected the exhaust system—no visible leaks or sounds of hissing. The pipe has slight blackness but not white, which is a sign of normal combustion. The engine idles well but isn’t perfect. I cleaned the PCV valve with solvent and now hear it rattle (I forgot to buy a new one during my visit to AZ). I cleared all codes and the vehicle has driven 26.8 miles since work was completed—no new codes have appeared. Typically, how long does it take for Ford vehicles to complete a drive cycle reset? I’m eager to return for the second emissions test but know it will likely fail due to the reset. Any insight would be helpful. Thanks!

The duration depends on how long it takes the engine control module (ECM) to complete a full drive cycle. This can take anywhere from 2 to 3 days in some cases. The most reliable method is using a scan tool that shows when all monitoring systems have passed—only after all monitors are completed does the system consider a full drive cycle complete.

Thanks, B. It’s been four days now and no new codes have appeared. Summary of work performed: 1) Replaced DPFE sensor — $49.00 2) Cleaned EGR valve intake port and intake O-ring 3) Inspected EGR hoses and manifold for leaks 4) Replaced both left and right bank upstream O2 sensors — $43.00 each (total $86) 5) Replaced PCV valve (initially cleaned, later replaced with a new one — $4.00) 6) Replaced intake air filter — $9.00 7) Cleaned the intake-side air sensor (uncertain of exact part name) 8) Inspected exhaust system — no leaks found 9) Checked downstream O2 sensor connections — secure and clean 10) Reset codes using a test unit — $79.00 (total cost: ~$230) Overall, the repair was manageable in cost and I now have a test unit for future use. I hope this 2002 F150 passes its next emissions inspection. The state offers a second free test, and the tag expires at the end of this month. P.S. Ford vehicles are notoriously difficult to pass emissions tests, and they don’t seem to hold up well over time. I may need to consider switching brands in the future. Thanks to everyone for your input! Kevin

I have a 2001 F150 and keep getting P0401 codes. I replaced both the DPFE sensor and EGR valve, but the check engine light remains on. The only possibility left is a vacuum leak—though I haven’t found it yet. I’ve used carb cleaner to test for leaks with no success. I’m lucky to have a reliable code reader that works across multiple vehicles. If anyone has advice or tips, I’d really appreciate it! Thanks!

Likely cause: a clogged port in the throttle body or intake manifold.