Questions and Answers

Toyota 4-Runner DTC P0171 Q. Dear Mr. Ciulla, I have a 1999 Toyota 4-Runner 6-cylinder Limited Edition. The Check Engine Light recently cam on, so I took it to a local AutoZone and had them run a diagnosis on the ECU. The code was a PO171 which means "Bank 1 too lean." I am not sure what that means, but I have a suspicion that it is the O2 sensor. If you can, please let me know if you know what this could be. Thank you, Tom

A. DTC P0171 is "System Too Lean (Fuel Trim) (Except CA)"

Fuel trim refers to the feedback compensation value compared against the basic injection time. Fuel trim includes short-term fuel trim and long-term fuel trim. Short-term fuel trim is the short-term fuel compensation used to maintain the air-fuel ratio at its ideal theoretical value.

The signal from the heated oxygen sensor indicates whether the air-fuel ratio is RICH or LEAN compared to the ideal theoretical value, triggering a reduction in fuel volume if the air-fuel ratio is rich, and an increase in fuel volume if it is lean. Long-term fuel trim is overall fuel compensation carried out long-term to compensate for continual deviation of the short- term fuel trim from the central value due to individual engine differences, wear over time and changes in the usage environment.

If both the short-term fuel trim and long-term fuel trim are LEAN or RICH beyond a certain value, it is detected as a malfunction and the MIL lights up.

When DTC P0171 is recorded, the actual air-fuel ratio is on the LEAN side. When DTC P0172 is recorded, the actual air-fuel ratio is on the RICH side.

The two most likely possibilities are a vacuum leak or a problem in the HO2S circuit or the HO2S itself. (Bank one Sensor one.)

It could also be a bad Mass Air Flow sensor or a bad Engine Coolant Temperature sensor.