What does it mean?
This code concerns the measured value rather than the wiring: the peak combustion pressure reported for cylinder 6 is lower than the control module expects for the fuel quantity, engine speed and load in use. The pressure trace is the module's direct window into how much work the cylinder is really producing, so a low peak points either at a cylinder that is not burning its charge properly or at a sensor that is under-reporting.
Weak compression, a lazy or blocked injector, missing ignition energy on gasoline engines and a soot-plugged sensor port all produce the same low reading. Because the module compares cylinder 6 against its neighbours and against a modelled target, it will normally have tried to compensate with extra fuelling before giving up and storing the code. The MIL is set and combustion feedback for that cylinder is disabled.
Symptoms
- MIL on, often alongside a misfire code for the same cylinder
- Rough idle with a distinct beat as one cylinder contributes less than the rest
- Noticeable loss of power and poor pulling under load
- Higher fuel consumption and, on diesels, grey or black smoke
- Slower starting and rougher running until the engine warms through
- Vibration felt through the pedals or steering wheel at low engine speed
Likely causes
- Low compression on cylinder 6 from worn rings, a burnt or leaking valve, incorrect valve clearance or a head gasket leak
- Injector for that cylinder delivering too little fuel through wear, deposits or a failing solenoid or piezo stack
- Pressure sensor port packed with soot, damping the peak the element actually sees
- Sensor under-reporting after drift, or fitted with the wrong sealing washer or at incorrect torque
- Weak spark on gasoline engines from a tired coil, a worn plug or a failing plug boot
- Leak past the injector seat, glow plug seat or head gasket bleeding pressure out of that cylinder
- Camshaft timing out by a tooth, or a boost or intake leak reducing the charge reaching cylinder 6
How to fix it
- Study the freeze frame first, then read the live pressure traces of all cylinders together to see how far cylinder 6 sits below the others
- Run a relative compression test, then a wet and dry compression or cylinder leak-down test on cylinder 6 to confirm or rule out a mechanical loss
- Check injector return volume, correction values and, where available, the individual cylinder contribution figures
- Swap the suspect injector or ignition coil with a neighbouring cylinder and see whether the low pressure follows the part
- Remove the pressure sensor or pressure sensing glow plug and inspect the tip and its passage for hard soot, cleaning or renewing as required
- Verify valve clearances and camshaft timing before condemning the sensor
- Repair the mechanical or fuelling fault found, refit the sensor to specification, clear the code and confirm the pressure trace matches the other cylinders under load