Today CARspec had a 2005 Toyota Prius with a rough idle, stalling and an illuminated check engine light. The check engine light pointed the technician to code P0301 – Cylinder #1 Misfire.

Interior of a 2005 Prius

The engine control module logged a rolling misfire (a consistent, repeated misfire) on #1, with a small count on #2. The technician started the vehicle while watching the misfires via the datalist and noted no current misfires on #1 or #2, but one count on #3. The vehicle ran smoothly during the remainder of the function check – no other misfires, good fuel trims, and a happy computer.

As with any malfunction code, duplication is the key to diagnosis – see it and you can fix it. The technician performed a visual inspection of the #1 ignition coil and spark plug, both appearing superficially functional. Relying on previous experience with this particular drivetrain (we here at CARspec have replaced many coils on the 2004-2009 Prius), the technician acted on a hunch, marking the #1 coil with a “?” and moving it to cylinder #2. Soon after starting the vehicle the car began misfiring repeatedly on cylinder #2. The #1 coil (now on #2) required replacement.

This diagnosis proved interesting because of the intermittent nature of the problem and inconclusive misfire data – a bit of luck and a lot of experience can help with diagnose a odd issue, especially when it came to the repair on this Toyota Prius.