P0171 fault code is back again... - Toyota RAV4 Forums
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#1 (permalink) Old 01-17-2013, 12:06 AM
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P0171 fault code is back again...

CEL came back on on my 98 Rav. I checked it with my Innova code reader and found out that I had P0171 again "System too lean". I had the same problem last May and posted it here. Fault Code P0171 and P1133

Last time the problem seemed to go away on its own after I cleared the fault, but not this time.
I pulled the spark plugs and found out that just like last time when I had this code was that the engine is actually running too rich. Freeze frame data showed that the STFT has gone +24% and the LTFT has gone to +44% meaning that the ECM is calling for full fuel. It is my understanding that when the short term fuel trim goes to it's max value, calling for fuel, then the fault code P0171 will be set.
This RAV4 has the Calf. emissions package so the first sensor is a A/F sensor.
I checked the A/F sensor's heater resistance and it was in spec. I would hate to go out and buy a new A/F sensor without being sure that it has failed. An opinions??

Last edited by 1rarefish; 01-17-2013 at 09:13 AM.
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#2 (permalink) Old 02-12-2013, 11:21 AM
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No reply's from any Toyota Techs ??

Just an update. After I cleared the fault last month. The fuel trims pretty much want back to normal and the vehicle ran fine. Well yesterday the fault code P0171 came back again. The freeze frame data showed that the LT Fuel trim was maxed out again.
I am now thinking of just replacing the AF ratio sensor on a guess that it might be the cause of this intermittent problem.
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#3 (permalink) Old 02-12-2013, 11:48 AM
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Can't comment on what the Innova is saying, we only have OBD1 over here but I do know that an engine temperature sensor is a lot cheaper than an A/F sensor. It's the larger of the two, two wires, they have been known to play up, it could be stuck and telling the ECM the engine is constantly cold, hence a rich mixture.
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#4 (permalink) Old 02-12-2013, 12:36 PM
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Quote:
Originally Posted by 1rarefish View Post
No reply's from any Toyota Techs ??

Just an update. After I cleared the fault last month. The fuel trims pretty much want back to normal and the vehicle ran fine. Well yesterday the fault code P0171 came back again. The freeze frame data showed that the LT Fuel trim was maxed out again.
I am now thinking of just replacing the AF ratio sensor on a guess that it might be the cause of this intermittent problem.
Before investing on new A/F sensor, try to see if the sensor is switching between (0.1V and 0.9V), The Innova 3130 or 3160 read live data, that is the only way to see the switching.

It does also check the ECT sensor (That Ken is talking about), This one should be around 185F.

Good luck.

Last edited by Sed1999Rav4; 02-12-2013 at 06:29 PM.
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#5 (permalink) Old 02-12-2013, 01:11 PM
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I have been some what concerned about coolant sensor. I noticed that when the code was set the coolant temp. was reading 158 degs. but I had not driven the car too far when the code was set. So the reading may be actually be correct. I have seen the coolant reach 194 degs when monitoring it.
When I let the car fully cool down. The Intake air temp pretty much matches the coolant temp when read on the Innova. That is a recommended way to some what check the temp sensors.
I really don't believe that it is the A/F sensor. It's voltage seems to change correctly when I rev the engine then drop it back to idle. I also believe that if it was the A/F sensor. I would be getting some additional fault codes.
I'm not able to check some of the other causes of the P0171 code. Such as fuel pressure and injector blockage.
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