If a vehicle has a lean condition, which fuel trim code might be expected?

In the context of engine performance and diagnostics, a lean condition refers to a situation where there is an insufficient amount of fuel in the air-fuel mixture that the engine is receiving. The fuel trim codes are diagnostic trouble codes (DTCs) that help identify issues related to the air-fuel mixture.

P0171 specifically indicates that the engine control module (ECM) has detected a lean condition on bank one of the engine. This code is generated when the fuel trim adjustments needed to maintain the correct air-fuel ratio are at a high negative value, meaning the engine is running lean and not enough fuel is being delivered relative to the amount of air. Thus, this code directly correlates to scenarios where there's a lack of fuel, either due to air leaks, fuel delivery issues, or sensor malfunctions.

The other codes relate to different conditions. For example, P0300 signifies a random/multiple cylinder misfire, which is not a direct indication of a lean condition. P0135 refers to an issue with the oxygen sensor heater circuit, which may affect air-fuel mixture readings but does not specifically indicate a lean condition. P0420 indicates a catalyst efficiency below threshold, which could be impacted by a lean condition but is not diagnostic