2011 Ford Fiesta High-RPM Misfire-P0302/P0303
The Problem
A 2011 Ford Fiesta with the 1.6L engine came in with a P0302 cylinder #2 misfire code. The engine ran normally at lower RPM, but at approximately 4,500 RPM it would begin to noticeably misfire. At times, a P0303 cylinder #3 misfire would also appear.
If the misfire continued long enough, the engine computer would disable the affected fuel injector to protect the catalytic converter.
Previous Repairs & Testing
The vehicle already had a new ignition coil and replacement fuel injectors. We verified engine compression was good and moved the spark plugs between cylinders, but the misfire remained associated primarily with cylinder #2.
Rather than continuing to replace parts, we investigated the engine computer’s crankshaft misfire detection strategy.
The Diagnosis
The Ford PCM detects individual-cylinder misfires by monitoring very small changes in crankshaft speed. For the system to accurately identify those changes, the PCM must have a properly learned crankshaft profile.
A conventional crankshaft relearn procedure was not available through the scan tool on this vehicle configuration.
We performed an engine parameter reset and then drove the vehicle while monitoring the crankshaft relearn PID with our diagnostic equipment. During the drive cycle, the PCM successfully relearned the crankshaft profile.
The Fix
Once the crankshaft profile relearn was completed, the high-RPM misfire indication disappeared.
The vehicle could now be operated through the RPM range without the previous high-RPM misfire or injector shutdown.
Why Proper Diagnosis Matters
This vehicle is a good example of why a trouble code doesn’t necessarily mean the component named in the code—or even the cylinder reporting the problem—is actually defective.
More parts could easily have been replaced without correcting this problem. Testing the system, understanding how the PCM detects the fault, and verifying the repair led us to the actual cause.
D & D Automotive – Tucson, Arizona
Diagnose the problem. Fix the cause. Don’t just replace parts.