CASE 003
P0301 Cylinder 1 Misfire
advanced
25 min
Petrol
Vehicle
- ModelTraining Sedan 1.5
- Engine1.5 TSI
- Mileage78,500 km
Customer complaint
Rough idle, slight shake and a loss of power. The check engine light is solid, not flashing.
History
Spark plugs replaced 3,000 km ago at another workshop. The fault returned.
Fault codes
P0301 Cylinder 1 misfire detectedP0300 Random misfire detected
Live data
- Misfire counter, cylinder 1Rising steadily
- Fuel trims both banksWithin a small band
- Coolant temperatureNormal
Measurements
- Cranking compression, cylinder 1Noticeably lower than the others
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Diagnostic case
Next step — Reasoning.
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Confirmed cause
Mechanical cause — low compression on cylinder 1 (for example valve sealing or piston ring wear), not ignition.
Reasoning
The misfire is isolated to one cylinder, fuel trims are normal and the ignition parts are new. A low cranking compression result on a single cylinder points to a mechanical cause. Isolate with a running compression test and a cylinder leak-down test before opening the engine.
Recommended repair
- Perform a running compression and cylinder leak-down test on cylinder 1.
- Inspect valves, seats, rings and head gasket as indicated.
- Repair and verify with misfire counters and a road test.
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.