Sirion - apparently misfiring on one cylinder


The malfunction indicator light on my partner's Sirion 1.3 auto (not long out of warranty - typical!) came on about ten days ago. She had it checked, & was told that the fault shown was a misfire on no. 2 cylinder. The mechanic who checked it couldn't find any fault in the ignition coils or the cylinder compression, or detect any fault in the engine when running. He changed the spark plugs, & swapped the ignition coil from no. 2 cylinder with another cylinder, & cleared the OBD memory. 4 days later, it came back. Same error, same cylinder.

My partner can't hear any unusual noise from the engine. There are two new minor & intermittent electrical problems (CD player cutting out & radio coming on, odd noise from parking sensor), but I've been told by the dealer she bought it from that they're on a different circuit from the ignition.

Any suggestions? How serious is the misfiring likely to be? Is there anything in the electrical system, or the engine control unit, which could cause this? Could it be a spurious fault, i.e. an error in the fault reporting rather than a real misfire? What should we do before resorting to having the cylinder itself looked at?

We're hoping we can sort it without spending a large amount having cylinders pulled apart.