CEL oddity

[oddyager]

A week ago the CEL came on my Ford Focus (2000, V4, 2.0L). Using the code reader it showed as a P0171 error (system too lean). After some research it recommended cleaning out the MAF sensor and if that didn't work replace it completely. It also checked to make sure there weren't any vacuum leaks from the air intake through the MAF towards the engine. There are definitely no leaks. Cleaning out the MAF sensor didn't do anything so I had it replaced. Wiped the code and then started it up fine. After about a day and a half of use, the CEL comes back on with the exact same code. Thinking the replacement MAF sensor was defective I had it exchanged. The same thing happens. Wiped the code and after a day and a half of usage the CEL comes back on. Typically if there is an issue the CEL should come back as soon as you startup, correct? Any thoughts whats going on here?

 

[lsd]

You should double check for vacuum leaks and then triple check after you're done. Vacuum leaks can be very hard to find. Could also be the o2 sensor or an exhaust leak somewhere between the head and o2 sensor.

 

[SoulAssassin]

Yeah, I was gonna say o2 sensor.

 

[brblx]

o2's probably fine since it's reporting in range values. i'd look for vacuum leaks, as stated above, and perhaps check for an exhaust leak near your upstream o2 (flex pipe is common).

 

[CurseTheSky]

I know it doesn't help at all, but I think you mean I4 (inline four cylinder), not V4 (V-configuration four cylinder).

 

[JeffKuhn]

The error codes might come back immediately after resetting and restarting the engine, or may take longer. I once had a vacuum leak that would take about 5 minutes to throw the code again. A bad O2 sensor on a Camry took about 5 minutes too. (That was setting the same code you are getting, but I also was getting a few others).

 

[exdeath]

Check things like PCV valve and oil cap, EVAP system, EGR system, MAP sensor, throttle body and idle control valve, etc. Anything that has a connection to the intake manifold, all the way up to the ports, not just hoses or the intake pipe.

Make sure the fuels system is in spec. You could get the same code with zero vacuum leaks by having a low battery or low fuel pressure or something as well.

Nothing suggests that the O2 sensors are working properly. They could easily be reporting on the leaner side and still be in range and not trigger an O2 sensor code. Easiest way to check would be to see if a sniffer agrees.

CEL doesn't come on right away, there is a certain amount of time that the problem must persist for the computer to know it's a definitive continuing problem and not a freak occurrence. Otherwise the light would be coming on 100 times a day.