OBD II Code P0171

[mhayden]

Hi,
Mine is Toyota Corolla 2000 CE, at 107K miles on it. My check engine light has been on and off for past couple of months. Finally,I got the code reader and read the code. It's P0171. There's just only this code. What could be the problem? Is it related to fuel injector? My mechanic advised me the Fuel Injection service 3 months ago during my last oil change. Your advice is highly appreciated.

Mine is mostly city driving. When the CEL is on, If I drive it on the freeway it goes off soon. But if I keep driving within the city it keeps on for days.

Thanks,
M

[pchalpin]

0171 is probably a worn out oxygen sensor.

[zuuma]

P0171 indicates a lean condition. Nothing more.
If it was accompanied by a P030x code [where "x" is the cylinder number], it would indicate a lean condition + misfire on the indicated cylinder.

so... your engine's computer is having a hard time keeping the correct fuel trim.
An aging (failing) O2 sensor could trigger P0171, but I'd place a higher probability that it is a dirty MAF sensor.
O2 reports the result after the burn. anything upstream of the o2 sensors - which is everything - will typically default to "rich" condition upon failure. That's because a slightly rich engine tune is not ideal, but a lean engine tune can detonate/ ruin an engine. Since your engine is lean, something is amiss, but not in failed condition.

Back to MAF. The MAF is located just downstream from the air cleaner. It reports the mass of the air going in. Very important for the computer to calculate the correct amount of gas to add via the injectors. If the MAF becomes dirty, it will mis-calculate the true amount of air entering the engine. In other words, more air is sneaking in than the computer can see. Just like glasses. Dirt obscures what it can "see", thus... a lean condition develops.. thus P0171 is tripped.
What to do?
carefully unplug the harness connector from the MAF, remove the two tiny allen screws holding it to the air intake pipe (watching out for the annoying throttle support wire/thing), ease it out and clean the little metal bulb-tipped thing.
Use a soft paper towel & some electrical contact cleaner (NOT carburator cleaner or Brake cleaner) & GENTLY swab away any grime.
Re-assemble & enjoy.
Best of all, this is a FREE fix. On a 100k+ mile car, you should do it anyway.
If I'm wrong, now you can shell out for a new O2 sensor. After 100-150k, they slow down anyway, so ok to change just for fun.

If you want odds, I'd say 75% dirty MAF, 20% O2 sensor, 4% failing MAF, 1% other.
Start with free, work your way up to expensive.
------------------
IF you get the P0171 AND a P030x code, that's another story. Coil packs & injectors may be involved. Not so in your case.

[teppista881]

Zuuma's is a good reply but you need to clean the hair-width wires inside the MAF sensor housing as well. The piece with the bulb is the idle air temperature sensor.

[zuuma]

YES!
I stand corrected. Clean both. GENTLY!