Intermediate
OBD Code Challenge
Engine Management
P0300 misfire under load
A petrol estate sets P0300 with a flashing lamp on the motorway. Use the freeze-frame, the trims and a fuel pressure test to find the shared cause.
- Peugeot 308 2015 · Petrol, manual · 1.2 PureTech
- Estimated duration: 25 min
- Pass mark: 60
- Yes — they reduce your score
Scenario overview
Customer complaint
“It shudders and the engine light flashes when I accelerate onto the motorway.”
Initial symptoms
Misfire under acceleration and at motorway speed. Flashing check engine lamp (severe misfire). Idle is slightly rough but acceptable. Fuel consumption has risen.
Initial observations
Spark plugs replaced 8 000 km ago by another workshop. No improvement.
Relevant history
Original ignition coils. Fuel filter never changed.
Scenario
The vehicle drives normally at low load and misfires under acceleration. Work out whether this is ignition, air or fuel before replacing anything.
Learning objectives
- Use freeze-frame data to target the diagnosis instead of replacing parts.
- Read fuel trims to separate an air leak from a fuel supply problem.
- Recognise that a shared cause explains a multi-cylinder misfire.
Diagnostic tools at your disposal
- Fault codes and freeze-frame — OBD scanner
- Fuel trims at idle and under load — OBD scanner
- Fuel pressure and delivery under load — Fuel system test
- Inspect the fuel filter and pump pick-up — Visual inspection
- Swap a known-good ignition coil — Sensor / actuator test
How you are scored
- Relevant diagnostic tests30
- Interpretation of results20
- Final diagnosis25
- Repair selection15
- Final verification10