Check engine light
The ‘check engine’ warning light is on in my 2004 MG ZT 2.5 petrol with 31,000 miles, and I have the fault code ‘P0300 (multiple random misfire) detected’. A specialist MG garage where I had just had new timing belts fitted said the fault could be the spark plugs, but I changed these (the old ones had only done 4000 miles) and the warning light is still on. Another garage could find nothing wrong and said it could be a sensor. The vehicle still runs smoothly and has a full service every 12 months.
Kev Brook
Given the code ‘P0300’, there are a few possible reasons for the problems aside from the spark plugs. These include the HT leads, coil packs and injectors or a wiring failure within the engine loom.
Another more worrying cause may be lowered compression. As you said that new timing belts have just been fitted, I would hope that the management light did not come on immediately after the job was completed. If this was the case, I would ensure that the three timing belts (two rear and one front) were correctly fitted.
Provided you can discount the new timing belts as being an issue, and as the spark plugs have been replaced, I would be looking into the other possibilities. Normally, the coil packs, injectors, etc, would cause a misfire that you would feel in operation. As this is not the case, I would look for either an air leak in the induction system or the possibility that the 02 sensor is reading incorrectly and instructing the ECU to weaken the mixture. The latter can be checked using diagnostic equipment and you should be looking for any imbalance in the long-term fuel trim figures. If the ECU appears to be continually weakening the mixture, it may be due to the 02 sensor. However, if the ECU appears to be attempting to richen the mixture, then an air leak in the induction system is likely.
If after the above checks have been carried out nothing is brought to light, I recommend using a diagnostics scanner to read off the live data, which should help you to pinpoint the source of the problem.