I decided it was time to replace spark plugs on my daughter's 2001 Tribute LX. All went fine in pulling the intake manifold and getting everything back together. The car started up and has runs well. However, after a few days the Check Engine light came on. The code indicates Bank 1 is running lean. A web search leads me to check the Mass Air Flow sensor and vacuum lines. I sprayed the MAS with contact cleaner, as instructed. All vacuum lines looked good. I sprayed car cleaner around the vacuum connections and manifold ports, but there was no change in idle. Doesn't appear to be any leaks.

Before I spend over $100 on a new MAF sensor, I'm checking if anyone has any other suggestions.

Thanks in advance.

 

If it were a 4 cylinder engine with only 1 bank then i would lean toward the MAF. However since it is a v6 which has 2 banks and should kick a code for lean bank 1 and 2, I highly doubt it is the MAF. Possibly the intake gasket is leaking between 2 cylinders if you don't find an external vacuum leak. Possibly a bad upstream bank 1 O2 sensor if not a gasket leak. Maybe it only has a vacuum leak when cold, once it warms up it goes away. Try the vacuum leak test when it's cold. Try a bottle of Techron in the gas tank to clean the injectors and decarbon the valves. It sounds like you are going to have to remove the intake again. It is possible that something got to the O2 sensor and contaminated it. Cleaner maybe or a bit of trash. They work off a chemical reaction so are easily destroyed with contaminants.

 

Thanks for your thoughts. I did a good soaking of several areas with carb cleaner while I had everything apart. There was build up on the throttle body and it got several heavy sprays. I'll turn my attention to the O2 sensor. I replaced all of them not to long ago. I would have thought the carb cleaner would have evaporated by the time I got the manifold back in place and started the car. But, who knows.

I'll also pull the intake manifold and visually inspect the gaskets. I did put new ones in.

 

You might try clearing the code and see if it comes back. O2 sensors can get lazy if contaminated. ( if it does not switch rich/lean within a ten second period it kicks a code) It might come out of it on it's own.

 

I did clear the code and it came back. I didn't remove the lower manifold when I replaced the plugs. What's the chance it's the lower manifold gaskets that are leaking? They were in the gasket kit I bought.

 

Unlikely but in the realm of possibility i suppose.

 

I've rediscovered there are 4 O2 sensors. Any thought on which one(s) are most likely to trigger the P0171 code?

 

It would be bank 1 sensor 1. Sensor 1 means upstream before any converter. The PCM only uses the upstream o2 sensors for injector dwell. The downstream O2 sensors only monitor the efficiency of the converters. Still,the code you have is very common and 90% of the time due to a vacuum leak. You said you replaced the O2 sensors not long ago. Do they have a warranty? Alternately,If the connectors on the O2 sensors are the same you could swap them to a different bank and see if the code switch's banks.