Skoda Fabia 1 1.2l petrol DOHC misfire - a cautionary tale.

If you are the sort of person who, like me, starts shouting at your screen when you read tales of stupidity... count to ten and try to read to the end.

This is a tale of how to *not* correctly diagnose a misfire problem and then recover the situation.

1) I have an OBD2 to USB cable and the VCDS software on my Windows laptop. Thing is, I very rarely use Windows these days so that's on a spare hard disk which I only swap in when I absolutely have to.

2) Years ago I had a Peugeot with a misfire problem which was traced by my garage to an ignition coil - the whole block of four had to be replaced. My Fabia misfire felt similar, not driveable, no power, lumpy as hell. Check Engine Light (CEL) was sometimes flashing or otherwise remained constant...

3) So I bought a cheap HAAS replacement coil for my Fab and started swapping. New coil on cylinder 1 (nearest the pulleys)... No good. Coil from cylinder 1 onto 2... No good. Coil from cylinder 2 onto 3... No good... Rats. It can't be a bad coil... OK swap the coil from 3 onto 1 taking out the new coil. Maybe one of the spark plugs or fuel injectors?

4) At this point I finally dug out my Windows hard disk and plugged in the diagnostics. Clear all the fault codes and then wait for new ones... Cylinder 1: Misfire Detected P0301 - 35-10 - Intermittent.

5) OK take all the plugs out and inspect - they look fine. Rats. Should have kept track of which one came from where... Put them all back and re-run diagnostics. Cylinder 1: Misfire Detected P0301. OK. Swap plugs cylinder 3 to 2, 2 to 1 and 1 to 3. Re-run diagnostics: Cylinder 1: Misfire Detected P0301. Hmm.

6) Fuel injectors: Swap 3 to 2, 2 to 1 and 1 to 3. Re-run diagnostics: Cylinder 1: Misfire Detected P0301. Oh hell, it's either the valves or the wiring loom.

7) Dig out my old compression tester... Damn, need an extension tube to reach down to the plug holes. Go buy that and run the test: all cylinders satisfactory just under 300psi. Damn. It must be the wiring - or the Engine Managment System (on-board computer)?

8) At this point my swearing attracted my engineer son's attention. So I explained that I'd swapped each coil, each plug and each fuel injector and that it's probably not the valves... He asks me to clarify so I demonstrate reading the fault code (Cylinder 1: Misfire Detected P0301) and then swap the coils between cylinder 1 and 2. Re-run diagnostics: Cylinder 2: Misfire Detected P0302. Wait... what?...

9) Lightbulb moment: Swap the new shiny HAAS coil onto cylinder 2. Re-run diagnostics: Cylinder 2: Misfire Detected P0302. Damn. Hours wasted.

10) So I buy a genuine VAG coil from my Skoda dealership and swap it on to cylinder 2. No more misfires, CEL goes out and stays off. Engine running as sweet as it ever did... Re-run diagnostics: no faults.

The lessons are: Change one thing at a time, use the diagnostics to check and don't buy the cheapest parts!

I hope this helps someone else - I certainly feel better now.