I have a '00 Expy 5.4L. Check engine light came on with P0171 and P0174. The parts store is telling me to change out the Oxygen sensors but I read something about cleaning the Mass Flow Sensor. I think I will try this cheaper route first and if the check engine light does not clear I will change out the sensors.

Does anyone have an other explaination to why both sensors would fault at the same time? That is what makes me doubt changing out the sensors?

 

Autozone is great for the free code scan. As soon as they start making suggestions on what parts to buy to "fix" your problem, run like hell. Autozone's in business to sell parts, not fix cars.

That said, you're thinking right, P0171/0174 does not indicated failed O2 sensors. 99 times out of 100, these codes indicate a vacuum leak somewhere, outside air entering the engine after the mass air sensor. Start with inspecting the PCV tube that runs from the passenger's side valve cover to the passenger's side and rear of the intake manifold, pay extra attention to the rubber elbows in this long hose. You've probably got a section of that hose rotted through somewhere.

 

Thanks Quintin. I found that the air inlet was loose and a bad rubber tube coming off of the exhaust in a pair into a sensor. CEL is off!

That sensor is the DPFE sensor. My tape fell off so I am replacing the the tubes with 1L3Z-9P761-AA and 1L3Z-9P761-BA