P0171

I know this code has been talked about more than any others but I still have a few questions. My CEL is on, Code P0171 System Too Lean (Bank 1). But it doesnt tell me sensor 1 or sensor 2. ( I cleaned the MAF, no change) I called Totoya to get a O2 sensor but they said the O2 sensor is behind the CAT and there is an A/F sensor in front of the CAT. A/F is $220 and O2 is $160. I dont know if I should buy both or which one to buy.

 

I would google the code with the make, model and engine type. I did this once and I found the resistance readings each O2 sensor should have. I read them all out, found and replaced the bad one and it's been good ever since. Good Luck.

 

If it were an O2 sensor, it would be the pre cat sensor. The post cat sensor just monitors the cat. Have you done any work to the truck recently that may have caused to code? Or did it just pop up? I had this code after I did my valve cover gaskets and cleaned the TB. Couldn't figure out what it was. I decided to spay some maf cleaner around my intake to check for vac leaks. It idled up when I sprayed around the TB. I replaced the TB gasket and the IAC gasket. Code was gone for good.

 

Mine started idling rough and then hardly had any power. Had it checked and got the same code. Cleaned the MAF and it's running better but engine light is still on so it might be several things. Chris

 

First check the intake for any vacuum leaks and repair them. If that doen't fix it, move on from there. You have a code that says the engine is running lean. It could be caused by a clogged fuel filter, a poorly pumping fuel pump, a bad mass airflow sensor, and a few other things that are less common.

If it's not an intake leak, buy a $50 scanner and look at the voltages out of the MAF and both O2 sensors (although the post-cat oxygen sensor should not cause a lean code). One of those sensors is probably not doing its job. If it's a 4 banger you have B1S1 as your pre-cat air fuel ratio sensor and B1S2 as your post-cat oxygen sensor. Both voltages should flucuate constantly.

My P0171 was caused by a bad air fuel ratio sensor that was stuck at .66v. As soon as I replaced it, the light was gone and the truck ran like a top.

 

I had this same code and found a vacuum leak - new hoses and problem solved - cost me $5

 

I just got the same code, im doing a tune up on it and changing the fuel filter....I was told to check for vacuum leaks.....

How do you check for vacuum leaks?
I was told to use soapy water but that doesnt sound like it would help with a vacuum issue....

Please tell me what to spray to use and where/what in the engine bay needs to get sprayed
what are the signs of a leak?

Thanks

 

Just because you have a lean code does not mean your A/F sensor before the cat is bad. It means that more air than fuel is getting burned in the cylinders. Vacuum leaks, bad sensors, MAF sensor error, weak fuel pump, dirty fuel filter.... All of these can cause a lean code.

Resistance readings for the O2 and A/F sensors are predominantly for the heater. The signal from the sensor is a voltage typically reading between 900mV down to 50 mV. Baseline is 450mV so if you measure the voltage with the engine off, that is what you should see.

That is partially correct. On the Toyota, the pre-cat sensor is actually an A/F ratio sensor, not an O2 sensor. The O2 sensor, which is after the cat as you mentioned, does simply monitor catalyst efficiency. Your idea on finding a vacuum leak is the most common method. Good point there!

 

I had that code and it ended up being the MAF Sensor. Now I have P1135 which apparently is the o2 sensor before the cat.

 

Thanks for all the input! I ended up taking out the MAF and cleaned it real good and cleaned the intake with Sea Foam and so far that has fixed the problem for me.

 

Glad to hear it OP. Keep everyone updated if anything changes.

 

That wasn't the OP, that was a 'thread crasher'. The OP has yet to respond after his initial post.

 

One method that I have seen work many times is to use Starting Fluid. Whilethe truck is running, spray the fluid around areas of the possible leak (vacuumhoses, manifold/intake seals and gaskets.) You will know when there is a leakbecause the engine RPM will increase when spraying the right area and will comeback down when you move away or stop spraying.

Just remember that this stuff is flammable and too much can be a bad thing.

You could also go out and spend hundreds on a smoke machine or pay a placeto use one.