P039D

What it means

P039D means the engine control unit has seen the cylinder 1 combustion or cylinder pressure vary by more than its calibrated limit from cycle to cycle, or from the expected value. It is only used on engines that measure or calculate in-cylinder pressure, so the exact method depends on the model and engine. The unit sets the code when the pressure signal or its calculated variation stays above the limit for long enough.

Symptoms

• Engine management light on
• Rough idle or uneven running, especially on cylinder 1
• Misfire or hesitation under load
• Reduced power or the car going into limp mode
• Poor fuel economy
• Hard starting or an unstable idle after a cold start

Possible causes

• Fuel delivery fault on cylinder 1, such as a leaking, sticking or clogged injectorlikely
• Ignition fault on cylinder 1, such as a worn spark plug or faulty coillikely
• Wiring or connector fault on the cylinder 1 pressure sensor circuit, or a related sensor, if fittedpossible
• Faulty cylinder pressure sensor or the signal it feeds to the control unit, on engines that have onepossible
• Mechanical problem on cylinder 1, such as poor compression, valve sealing or a worn piston ringpossible
• Carbon build-up or an air leak affecting cylinder 1 charge, such as an intake valve deposit or an inlet manifold leakpossible
• Outdated or faulty control unit software or calibration, where the manufacturer has issued a fixrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data. Note engine load, speed, temperature and whether misfire or fuel trim codes are also present.
2. Check for technical service bulletins or software updates for the specific model and engine.
3. Inspect the wiring and connectors at cylinder 1 (injector, coil and any pressure sensor) for damage, corrosion, water ingress or chafing.
4. Check live data for cylinder 1 misfire counts and compare with the other cylinders. Also look at fuel trims and the pressure or combustion data if the engine provides them.
5. Remove and inspect the cylinder 1 spark plug. Swap the coil with another cylinder to see whether the fault follows it.
6. Test the cylinder 1 injector (resistance, balance or leak-down test) and swap it with another cylinder if the design allows.
7. Check for inlet air leaks near cylinder 1 and inspect intake valve condition where access allows.
8. If the fault remains, carry out a compression or leak-down test on cylinder 1 and test the pressure sensor circuit following the manufacturer's procedure.

How to fix

• Replace the worn spark plug or faulty ignition coil on cylinder 1.
• Replace or professionally clean a faulty cylinder 1 injector.
• Repair damaged wiring or connectors in the affected circuit.
• Replace the cylinder pressure sensor if testing shows it is faulty, if fitted.
• Repair any inlet air leak, and clean carbon deposits from the intake valves if they are the cause.
• Repair the mechanical fault on cylinder 1 if compression is poor, or install a manufacturer calibration update where one exists for this fault.

Can I keep driving?

Nearby codes

• P039CGenericCylinder 1 Pressure Variation Low
• P039EGenericCylinder 1 Combustion Performance