What does the P1204 code mean?
The P1204 code is a manufacturer-specific OBD-II diagnostic code that indicates a misfire or a fuel supply problem in cylinder No. 4 of the engine.
A misfire means that the air-fuel mixture in this cylinder does not ignite properly, which can reduce engine power, increase fuel consumption, and cause vibrations.
Possible symptoms of the P1204 code
🔸 Check Engine light on
🔸 Engine running rough or irregularly
🔸 Loss of power or reduced acceleration
🔸 Increased fuel consumption
🔸 Engine vibrations or shaking
Possible causes of the P1204 code
✅ Ignition problem
- Faulty or fouled spark plug in cylinder No. 4
- Faulty or shorted ignition coil
- Damaged ignition coil wiring harness (cut wires, corroded connectors, poor contact)
✅ Fuel supply problem
- Clogged or faulty fuel injector in cylinder No. 4
- Insufficient fuel pressure (fuel pump, clogged fuel filter, faulty pressure regulator)
- Leak in the fuel circuit
✅ Air intake problem
- Vacuum leak in the intake manifold
- Dirty or faulty Mass Air Flow (MAF) sensor
- Faulty Manifold Absolute Pressure (MAP) sensor
✅ Engine mechanical problem
- Damaged exhaust or intake valve
- Faulty head gasket causing loss of compression
How to diagnose and repair the P1204 code?
1. Check Technical Service Bulletins (TSBs)
🔎 Before any intervention, check if there is a TSB (Technical Service Bulletin) for your vehicle. Some models have known issues with the ignition or injection system that could explain this error code.
2. Visual inspection of the wiring harness and connectors
- Check the wires and connectors of the ignition coil and fuel injector for cylinder No. 4.
- Look for broken, burned, or corroded wires.
- Check if the connectors are properly plugged in and without bent pins.
3. Test the ignition (spark plug and coil)
✅ Replace or swap the spark plug from cylinder No. 4 with one from another cylinder.
➡️ If the problem moves, then the spark plug is faulty and must be replaced.
✅ Do the same with the ignition coil.
➡️ If the problem changes cylinders, then the coil is the cause and must be replaced.
4. Check the fuel injector for cylinder No. 4
- Disconnect the injector and measure its resistance with a multimeter.
- Check that it sprays correctly (requires a test with a diagnostic tool).
- If the injector is clogged, clean it with a suitable product or replace it.
5. Check fuel pressure and air leaks
- Use a pressure gauge to check if the fuel pressure meets the manufacturer’s specifications.
- Look for vacuum leaks around the intake manifold using brake cleaner (if a leak is present, the engine RPM will change when sprayed at the leak location).
6. Test engine compression (if necessary)
- If everything seems fine but the problem persists, perform a compression test on cylinder No. 4.
- A drop in compression may indicate a faulty head gasket, burnt valve, or worn piston rings.
Common repairs for the P1204 code
🔧 Replacing the spark plug in cylinder No. 4
🔧 Replacing or repairing the ignition coil for cylinder No. 4
🔧 Repairing or replacing the electrical wiring harness
🔧 Cleaning or replacing the injector for cylinder No. 4
🔧 Checking and repairing vacuum leaks in the intake
🔧 Engine repair if a mechanical problem is detected (valve, head gasket, etc.)
Common mistakes when diagnosing the P1204 code
❌ Not checking spark plugs and coils before replacing expensive parts
❌ Ignoring an injector or fuel pressure problem
❌ Not checking for vacuum leaks, which can cause misfires
❌ Forgetting to test engine compression if the problem persists
Conclusion
The P1204 code is usually caused by an ignition, fuel supply, or air intake problem in cylinder No. 4.
➡️ In most cases, a simple replacement of the spark plug or coil solves the problem.
➡️ If the ignition is good, check the injector and fuel pressure.
➡️ If the problem persists after these tests, consider a more in-depth engine inspection (compression, valves, etc.).
🚗 Need help identifying the exact cause? A diagnostic scan at a repair shop can save you time and avoid mistakes!