Engine Misfires — How Shaw’s Diagnoses Them When the Code Says Random or Multiple Cylinder
A P0300 code tells you the engine is misfiring. It does not tell you why. Here is how Shaw’s investigates the cause when the obvious checks have already been done.
A misfire fault code — particularly P0300, logged as random/multiple cylinder misfire detected — is one of the most common fault codes and one of the hardest to resolve. By the time a vehicle arrives at Shaw’s with this code, the straightforward checks (spark plugs, coils, leads, injectors) have often already been done. The misfire is still there.
What causes a misfire
A misfire is an incomplete or absent combustion event in one or more cylinders. The causes fall into three categories: ignition (spark plug, coil, lead), fuel delivery (injector quantity, fuel pressure, control signal), and mechanical (low compression, burnt valve, camshaft timing error, air leak creating a lean condition). A random misfire code means the ECU detected variation in crankshaft acceleration between cylinders — it is measuring an effect, not identifying a cause.
Crankshaft position signal analysis
The crankshaft position sensor is the reference the ECU uses to detect misfires. A sensor with a developing fault — intermittent signal loss, a damaged tone ring tooth, a loose mounting — can trigger misfire codes that have nothing to do with ignition, fuel or compression. The oscilloscope shows the actual waveform from the sensor rather than an interpreted value, which means faults that have not yet triggered a sensor fault code are still visible.
Engine cycle analysis
Cylinder-by-cylinder analysis across multiple engine cycles identifies which cylinder is misfiring and what the misfire pattern looks like. A misfire occurring on every cycle of the same cylinder points to a consistent mechanical or electrical fault specific to that cylinder. A misfire that rotates between cylinders suggests a fuel supply issue or crankshaft trigger fault. A misfire that appears under load but not at idle suggests a mechanical issue — low compression becomes more apparent when the engine is working hard. Advanced misfire diagnosis is carried out at our Rhyl branch. Diagnostic assessment: £150.
Contact Shaw’s about misfire diagnosis →
We check it properly. We explain it properly. We do the job properly. You decide what gets done.
Shaw’s Service Station | Rhyl & Kinmel Bay | 01745 353481