Hello I have a 1999 V6 Toyota Solara and trying to fix the Check Engine Light The Auto Zone Code Checker says the following: Troubleshooting P0171 OEM Brand: Toyota Definition Air/Fuel Mixture system lean bank 1 (Cylinder Number 1 denotes bank 1) Probable cause #1.- Low fuel pressure #2 - Faulty "MAF/VAF sensor" #3.- Cylinder misfire condition #4 - Faulty H02S/AF sensor #5 - Large Vacuum leak First I replaced the downstream O2 sensor and both upstream O2 sensors but that did not help. The exhaust was leaking so, I had the front flex pipe before the catalytic converter replaced which was before the down stream O2 sensor. That helped and no light when driving around town for a couple for days, but the check engine light came back on when driving early on a cold morning on the highway. The Toyota parts man said their front flex pipe is part of the catalytic converter and is an over $2k install. My mechanic is really a super guy, he installed just the flex pipe for $150 !! The Toyota parts man also said that if the exhaust was a problem with the check engine light then there should be an additional 420 error code and in my case there was no 420 error code!! Nice vid on MAF - Mass Air Flow Sensor http://www.youtube.com/watch?v=sSuL58YEH-E The Answer appears to be: If it is not a vacuum leak then the MAF - Mass Air Flow Sensor or A/F Air/Fuel Sensor if there is a P1135 error code also! I also found this from http://www.toyotaownersclub.com/for...4e90d4c548e9b947301feb65&showtopic=9206&st=15 Posted 11 August 2004 - 12:25 AM I can almost guarantee you that your PO171 code is because of a bad Mass Air Flow Sensor (MAF). I work at a Toyota dealership and we see this problem alot and 99 times out of 100 it is a bad MAF. Hope this helps. Posted 14 October 2004 - 12:24 AM - This refers the the Air/Fuel A/F sensor not the Mass Air Flow (MAF) sensor This A/F sensor and computer problem is widespread. Toyota probably does not want to admit it. There probably is a malfuction in the match up between the computer and the sensor (feedback loop) that is preventing the air and fuel ratio to be mixed properly. I would also venture to speculate that this problem involves other Toyota products other than Camry, that use this same subsystem for fuel delivery. I have to say that the dealership where I take my car was very fair and replaced both the A/F sensor and computer. It turns out that the 97 Camrys have an old version computer that is related to the problem. When I spoke with the master mechanic he told me that after taling with the factory, Toyota's recommended fix was to replace both the sensor and computer (over a $1000 in parts alone). Much appreciated was that he got the parts to be covered by Toyota and I just had to pay for the labor only. When this problem first occured I was under 100k but am now at 104k. This was the third time I had brought the car in and had already spent over $300 in labor. While I was waiting in line again the service person at the front desk was on the phone describing to the customer that they had a check engine light on with a lean condition, and that they were going to try to clean the A/F sensor........"Yes maam.... that would account for the split second hesitation in power.......... The master mechanic said that one of the other mechanics had my same car (97 Camry) and he too had the exact same problem - hesitation in power, followed by check engine light, followed by lean condition codes. Also said that replacement of the computer and sensor resolved the check engine light from coming on but he still gets that split second hesitation in power. Notes from the service write up: Code P0171 (system too lean) code P1135 (A/F Sensor heater circuit malfunction bank 1, sensor 1). Tech tested A/F sensor on scan tool and sensor is not responding to lean command. Tech prelaced ECU and A/F sensor. Hope this information helps. This might end up in the news at some point as a product recall due to defective design. I do not see any vacuum leaks so I am getting a new MAF tomorrow. Did anyone have the same error code and know what caused it?