I own a 2001 f-150 super crew. My service engine soon light just came on so I took it to auto zone, and the diagnostics test read that the 02 sensors on bank 1 and bank 2 are bad. I can't decide if i should replace the o2 sensors or try to replace the mass flow air intake sensor first. I need to know if the mass flow air intake sensor could throw off the reading if it is bad. If any one has any ideas give me shout.

 

I would get the codes reset first and see if they come back, for both banks to set codes at the same time is rather different. Did you happen to just fill up at a station that just took delivery of fuel. Here in the north that sometimes causes the water(condensation) in the station's tank to get mixed up in the fuel and then we pump it into our vehicles. Or was your tank about empty and you got a slug of condensation into the injectors.

Also possible to have a manifold leak, but that would have to be both banks having a leak.

 

What were the actual code number? If it's trouble codes 171 and 174 you have the same problem a lot of people have. P0171 is System lean bank 1 and P0174 is system lean Bank 2.

There are mainly 3 things that could set those codes. A dirty or faulty MAF sensor, a vacuum leak, or a dirty fuel filter. The car is basically telling you it's getting to much air(Vacuum leak) or not enough gas (Dirty Fuel Filter) or the MAF is giving the computer a false reading which in turn makes the computer adjust the fuel mixture wrong which both O2 sensor detect after combustion and set's the trouble codes.

Note: I'm assuming you have trouble codes 171 and 174. The safest approach would be check your fuel filter, clean the MAF with MAF cleaner, then get the vehicle vacuum checked of obvious leaks if you don't see a leak it might be a good ideal to find a shop who has a smoke machine, then if you are 100% sure there is no vacuum leak then consider replaceing the MAF sensor.

 

Just go straight to the PCV valve hose elbows. They're, by far, the most common cause of this pair of fault codes.

If the PCV valve hose elbows are not the problem, follow troubleshooting for other vacuum or intake leaks post-MAF.

The other stuff is pretty far down the probability list.

Steve