pchouston

I have a 2002 Lexus RX300 with 79,000 original miles on it and lately the check engine lights been on. Check with autozone and first I got error code p0141. I replaced O2 sensor on down stream, after the cat and that fixed the problem.

Now, I'm getting p0171 and p0174. I've read several forums about this issue, but if someone can guide me the the source that would be a help. Thanks.

Lexmus

Those codes are indicating a lean condition in both bank 1 and bank 2 of your engine. Thus your system is measuring less air intake than is actually going to the engine. This means you either have a bad MAF sensor that is incorrectly measuring the air intake, or there is some opening after the MAF sensor position that is allowing unmeasured air to enter the engine.

Look for any cracks or in the hose, or a problem where the hose connects to the throttle body. If nothing obvious is wrong there, I would change the MAF sensor, especially if it has never been changed before, or was changed to a non-Denso or non-OEM sensor. The part is a Denso 197-6020, and I had to change mine to fix a similar error.

pchouston

pchouston

Lexmus, thank you for directing me to the right direction. It's fixed now, running good as ever with no check engine, VSC, and track control lights. First I changed out the O2 sensor, under the driver side, after the cat. Next I took the MAF sensor and sprayed it with CRC sensor cleaner. Next I replaced air filter and used a better hose clamps on the air filter box. There a small hose behind the air filter box that seems to come undone every time the cover is taken off. That also triggers the lights on the dash, so I clamped them tight. Next took off the plastic air intake on the throttle body housing and made sure they were cleaned and had no cracks on the hose itself. Next took some CRC throttle body cleaner and spayed half the can cleaning the black sludge build up inside the throttle body. Both side, it's got double 3" throttle body. Next while all the covers were off, I check for rubber hose on all vacuum lines. All looked good since it's always been garaged. Last I put everything back together and reset the batteries. I waited for 20 minutes and fired the car up. Wha-La, no lights.

I've been driving this for about two weeks and no lights at all. Man, so happy now...the car looks great, car runs great, smoother engine idle, better gas mileage, faster rev and pick up. Thank you guys for the inputs and great forum. Over and out, until next time, hopefully not too soon.

Lexmus

Good news.

I recommend checking your fuel trims to see if they are high even though you no longer are getting a CEL. The P0171 and P0174 codes are triggered when the trims are over 25%. You could still have abnormal fuel trims and not exceed 25%. When I replaced the MAF, cleaning didn't work, and the bank 1 long term trim was around 25% and bank2 long term trim was around 20%. Thus only the P0171 code was triggered even though both banks were abnormal.

I can monitor the trims when necessary using a cheap bluetooth OBD2 reader from Amazon and the Torque app for android.

pchouston

Thanks again for your input. So if the fuel trims are over 25%, it will trigger codes? If the fuel trims are under 25% than the lights will not come back on? On the 2002 Lexus RX300, what is consider normal fuel trim %? I don't have any technical manuals so I wouldn't know. Also how would I be able to make adjustments to the fuel trim or is this done automatically by the ECU on the Lexus? Thanks again.

Lexmus

The 25% number is my understanding when I was leaning how to fix my P0171 code and was consistent with the fuel trim data I saw on my car prior to the MAF change.

Ideally the long term trims are near zero and the short term does fluctuate but around zero. But I think they should be under 10% in total. Fuel trims that consistently greatly deviate from zero means you have an issue with a sensor or the fuel system. You can't adjust the trims directly, they are the result of all the data from the sensors being sent to the ECU.

My thought is, you improved the MAF with the cleaning, the code went away, but it might still be wounded. And running lean is not good for the engine. This would be a good time to get some experience looking at the trims by getting the inexpensive equipment to do so, and understand how your car is truly performing given the codes you previously had.