Intermittent misfire one side

Paul Ritland · 10:57 AM

I have a 2005 Boxster S with some 48000 miles on it. In 2020 with 47000 miles I got a CEL indicating need to take it in for service. It was idling rough but seemed to be running well at speed. My OBD 2, indicated an intermittent misfire on one side. I took it for service and the coils and spark plugs were replaced. The apparent misfire at idle disappeared. In December 2022 the CEL indicated need for service and the OBD 2 gave me the same message, yet the idle was still smooth. The mechanic was unable to diagnose and cleared the reading telling me to return if the CEL returned. It returned in January 2023. My OBD again indicates an intermittent misfire on the same side. Before I return it to the mechanic, can anyone explain why this is happening?

JFP in PA · 01-27-2023, 12:30 PM

It's misfiring again

Paul Ritland · 02-01-2023, 07:43 AM

Codes are P0300, 301, 303, 304, 305, 306. Random/Multiple Cylinder Misfire Detected (Generic). No longer indicating on one side only. I'm afraid to drive the car, thinking that some damage could occur, for example, to the catalytic converter if the problem was related to O2 sensor. Before I go back to the mechanic, I would like to have ideas from the wider community, since he was unable at first to give me any solution.

JFP in PA · 02-01-2023, 07:44 AM

I'd start by looking at the fuel trims and MAF signal.

jlong · jlong's Garage

I agree here that the nature of the issue being reported over 5 cylinders is less likely to be a coil pack or spark plug issue. I concur with JFP, it could relate to the MAF (mass air flow) sensor not sending the correct airflow information to the ECU so that the fuel can be "matched" appropriately when injected into the cylinder thus causing a misfire due to the imbalance (or stated another way: the engine is "hunting" for the proper balance of air/fuel mixture).