P2178 Subaru Fuel Trim System Rich at Cruise or Accel

Possible causes

– High fuel pressure
– Leaking fuel injector or pressure regulator
– Failed Mass Air Flow (MAF) sensor

What about the obd code?

The Engine Control Module (ECM) has detected a rich air/fuel ratio

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P2178 Subaru Description

The Engine Control Module (ECM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. Fuel delivery is controlled differently during Open Loop and Closed Loop. During Open Loop, the ECM determines fuel delivery based on sensor signals without heated oxygen sensor (HO2S) input. During Closed Loop, the ECM adds HO2S inputs to calculate the short and long term fuel trim (FT) adjustments. If the HO2S indicates a lean condition, the fuel trim values will be above 0 percent. If the HO2S indicates a rich condition, the FT values will be below 0 percent. The short term FT values change rapidly in response to the HO2S voltage signals. The long term FT makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1.