P039B

What it means

P039B means the engine control unit has judged that the pressure in cylinder 1 is higher than expected. How this is measured depends on the engine: some use an in-cylinder pressure sensor, while others estimate it from crankshaft speed, knock sensor or other signals. Excessive pressure can point to abnormal combustion such as pre-ignition, or to a fault in the measuring circuit.

Symptoms

• Engine warning light on
• Pinking or knocking noise under load, mainly from cylinder 1
• Rough running or misfire
• Reduced power or the engine going into limp mode
• Poor fuel economy
• Hesitation or uneven idle

Possible causes

• Abnormal combustion (pre-ignition or knock) from carbon deposits, poor fuel quality or low octane fuellikely
• Ignition timing too advanced, or wrong or worn spark plugs (incorrect heat range or gap)possible
• Lean mixture on cylinder 1 from a faulty injector, intake air leak or fuel delivery problempossible
• Excess boost pressure on turbocharged engines (wastegate, boost control or sensor fault)possible
• Faulty cylinder pressure sensor, or damaged wiring or connector, giving a false high readingpossible
• Mechanical change raising compression, such as heavy carbon build-up, oil ingress into the cylinder or an internal engine faultrare
• Outdated or faulty engine control calibration, where the manufacturer has issued a known updaterare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting load, rpm, temperature and boost when the fault set.
2. Check for related codes such as misfire, knock sensor, fuel trim, boost or timing faults, and deal with those first.
3. Ask about recent fuel used and any recent engine work, tuning or part changes.
4. Inspect the wiring and connector of the cylinder pressure sensor (if fitted) and the knock sensor for damage, corrosion, water or oil.
5. Inspect the spark plugs and coil for cylinder 1 for correct type, wear, deposits or signs of overheating.
6. Check live data for fuel trims, boost against target, ignition timing and knock retard, and look for intake air leaks.
7. Test the cylinder 1 injector operation and spray pattern or flow, following the procedure for that engine.
8. Carry out a compression or leak-down test and a borescope inspection of cylinder 1 for deposits, hot spots or damage, then check for any applicable calibration update.

How to fix

• Repair or replace damaged wiring, connectors or a faulty cylinder pressure sensor.
• Replace spark plugs or ignition components with the correct specification.
• Repair air leaks, or fix the injector or fuel supply fault causing a lean mixture.
• Repair boost control faults, such as the wastegate, actuator or boost sensor, so boost matches target.
• Clean off carbon deposits using the manufacturer's approved method, and use the correct fuel grade.
• Apply a manufacturer calibration update if one exists for this fault, or carry out the necessary mechanical engine repair if testing shows internal damage.

Can I keep driving?

Nearby codes

• P039AGenericCylinder 1 Pressure Too Low
• P039CGenericCylinder 1 Pressure Variation Low