Bocatrip

My check engine light came on a number of weeks ago. My Indie pulled error codes P0171 and P0174 which are "system too lean for bank 1 and bank 2". I do 95% city driving. The check engine light came on after some extensive highway driving of about 100 miles. Now after 2 weeks of all city driving, the light went out on its own today. I was told by 2 sources that more than likely the Mass Air Flow sensor was a probable cause for the first incident of the CEL. I have not done anything nor changed any parts since. The only thing that occurred was I went back to all city driving. I thought the CEL had to be reset manually to go out. How or why did the CEL go out on its own? Any feedback would be greatly appreciated.

NL5

They go off after a set amount of starts. Usually, it will trip again after a short while. The codes will still be in the system if they weren't cleared by a scanner. Doesn't mean the problem went away.

Bocatrip

Thanks NL5. I had not heard of the light going out after a certain number of starts. I was under the belief that it stays on regardless. I do understand that my problem did not go away on it's own. The reason for the thread was I had thought the CEL had to be cleared before it would go out. Thanks for the feedback.

rkw77080

Here's what the repair manual says about the check engine light...

Bocatrip

Thank you RKW as you have confirmed what NL5 said about the light going out after 3 attempts to go on another trip. Unfortunately, it does not answer my concerns about what really was present to trip the first check engine light incident. As you mentioned if the fault is no longer detected the CEL will go out after 3 trips. My question is why is the fault no longer detected? What changed? Hmmmm. I'm going the route of changing the MAF within the next week or so.

NL5

The problem may be intermittent. I have a diesel truck with a sticky egr valve. Sometimes it trips right after the cel goes out, other times it may take 10 or more trips to reoccur. If it was a one time problem, you may be fine, but more than likely, it will trip again. It is my understanding that the codes you listed are often times MAF related. The are fairly cheap to replace, and is something most people can easily change themselves.
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Bocatrip

thanks. hopefully the MAF is the answer.. Thanks again.

biacs

My recent error cleared as well. You can check a few things - Make sure you have good gas. I had some mid grade recently so I topped off with Premium. Retighten the gas cap. Also I removed airfilter and small hose off the filter housing and re-attached. Im sure cold air can do weird things to our cars electronics and seals

Get an auto shop to read the codes with a scanner. Just use your phone to take pics of each one - then report back.

coug66

just putting my vote in for diagnosis, ( fuel pump)

its either too much air or not enough fuel.

My guess its low fuel pressure.

rkw77080

Here are a few other items that can cause P0171 and P0174...