Originally Posted by Yung_Ji

Hey Guys! Hope your week is going well.

Recently, I cleaned my MAF Sensor and Throttle Body as a preventative measure on my 2002 4Runner with \~148K miles (California). Before I did, I have never had a CEL on. The truck ran well but 3 days after, while I was accelerating on to a freeway that is on an incline, I saw the CEL come on. The truck drove fine even afterwards and I would not have noticed anything was wrong if the CEL did not come on; no crazy rough idling, etc, but maybe a bit weak on the throttle response at high speeds.

I ran the code and got **P0171 - System Too Lean (Bank 1).** I’ve done a lot of research here on the forums and it seems that most of the issues with this code comes from MAF Sensor issues and I did just get the MAF Sensor cleaned so I thought maybe the ECU needed resetting so I disconnected and reconnected the battery. The CEL went away for about a week (~70 miles of driving) and at the exact spot on the freeway entrance, the CEL came on again.



When I took it to the my mechanic at RPM Garage in Monrovia, CA who performed the MAF Sensor & Throttle Body Cleaning, they said that they could not find any clear sign of major vacuum leaks but the LT Fuel Trim reads high around 35\~38 under load, accelerating on a hill. It is their opinion that it is not the MAF Sensor that was recently cleaned but problem with the Fuel Pressure Regulator and the Fuel Filter, which were still factory.


The mechanic who also owns a 3rd Gen 4Runner said that he had the same issue when he cleaned his MAF Sensor, and he said that the computer can start to freak out when the readings are so different after a dirty MAF Sensor has been cleaned. I don’t have an unlimited budget so I am trying to figure out the next sensible move to fix the code? Should I get a new OEM MAF Sensor or try the OEM Fuel Pressure Regulator and Fuel Filter as my mechanic suggested?


Thanks a bunch!