Hi, My Mini R56 gave off a P0303 code over a week ago. I pulled the plugs and after almost 12,000 miles they were worn down quite far. I installed a new set of plugs, cleared the OBD error and drove the car but the error came right back after a short distance. Pulled the plugs back out the No 3 plug was clean. I connected the coil and plug from cylinder #3 into the #1, 2 & 4 connectors and got spark on those but not No 3. Is there any chance the severely worn plug could have damaged the ECU? I unplugged the battery for a couple of hours too just in case I might clear something else...

Thanks

 

If you put #3 coil in another cylinder and ran the car and it still had #3 misfire fault you more likely have a cylinder problem. I'd do a compression and leak down test to make sure. Also in rare cases carbon build up on cylinder 3 intake could cause it, but rare.

 

I would check the wiring at the connector for the coil. You could also do a comparison resistance check to determine if there is a problem in the circuit. I.E. compare resistance reading between a known good circuit to the #3. Although not impossible it is highly unlikely that it is the actual ECU itself. A more definitive circuit test would be to check between the coil connector and the ECU connector to determine if there is a broken wire. However, to do so you will need a wiring diagram.

 

Just wanted to mention the search function brought this topic up - P0303 error. Cleared the error with my cheap ODB reader, came back right away. Swapped the coils between the two middle cylinders (of course leaving the wires going to the same cylinder as before) and it came back with a P0302 error which tells me I had a bad coil - the problem followed the coil from cylinder #3 to #2.

$52 spent at an auto parts store on a replacement Duralast coil and all was good. Saved myself $100's of dollars at the dealer on my Justa.

Now the OP did the same test and found that the problem did not follow the coil or the plug, a whole 'nother situation where something else had to be done, but I wanted to say sometimes it really is this simple to fix this code yourself in minutes.

 

Yep sounds like the coil might not be getting any voltage from the wiring harness, check continuity.

 

Just checking, but P0303 is cylinder 3 misfire? That is very different from "no spark".

Just for clarification, are you hearing a misfire, is this constant or intermittent? Does the car seem to run ok, or is it running on 3 cylinders all the time? If in doubt, unplug the coil from cylinder 3 and drive the car, is that the same or different?

No spark implies there is no current getting to the plug. As this has not changed with a new plug and coil, and with a known good plug and coil, then this is most likely wiring related.

However, if it is an intermittent misfire that has not corrected with a good plug and coil pack, then this is whole different ball game. This could well be something internally wrong inside that cylinder, valve burnt out etc etc