A 2001 Subaru Forester L with 85,000 miles experienced intermittent hesitation under load, primarily when the engine was warm. The issue had persisted for a year, with previous diagnostic trouble codes (DTCs) indicating cylinder 2 and 4 misfires (P0302 and P0304). Initial attempts to resolve the problem included replacing spark plugs and wires eight months prior, but the issue remained. A dollar bill test over the exhaust pipe showed no restrictions, and misting the coil revealed no sparks. A fuel filter issue was deemed unlikely due to the problem occurring only when the engine was warm.
Recently, while cruising at 65 mph without accelerating, the hesitation recurred, and a new DTC, P0172 (System Too Rich Bank 1), was logged. Consultation with an auto parts specialist, along with a description of the symptoms, led to the suspicion of a faulty front oxygen (O2) sensor. The front O2 sensor had been previously replaced at 40,000 miles.
A new OEM front O2 sensor was purchased and installed. The replacement process took approximately one hour, accessed from the top of the engine bay on the passenger side. Removing the air channel assembly provided improved access. Thread lubricant was applied to facilitate future removal. The total cost for the part was $100.75.
Following the O2 sensor replacement, the hesitation problem was resolved, and the vehicle’s performance returned to normal. While the parts specialist indicated the sensor should last for 100,000 miles, this experience suggests a shorter lifespan may be possible, particularly with this specific engine model. Regular monitoring of engine performance and DTCs is recommended.
