What does it mean?
With a usable pressure trace from cylinder 3, the module measures the peak reached during each power stroke and compares it with the value predicted for the current engine speed, load, charge air and injected fuel quantity. This code is stored when the measured peak stays below the calibrated lower limit, meaning cylinder 3 is producing less pressure than the model requires.
The electrical signal is inside its normal range here, so the diagnosis moves toward mechanical condition, fuel delivery to that cylinder, or a sensor that reads low. A cylinder that cannot build pressure delivers less torque than its neighbours, and contribution or balance faults are frequently stored alongside this code. The MIL (check engine lamp) comes on and many applications reduce power until the condition is corrected.
Symptoms
- MIL (check engine lamp) on, usually with contribution, balance or misfire codes for cylinder 3
- Rough idle with a repeating beat the driver can hear and feel
- Poor cold starting and grey or white smoke during the first minute of running
- Clear power loss and slow response when accelerating under load
- Higher fuel consumption as the module compensates for the weak cylinder
- Vibration through the body at low engine speed and limp mode on some applications
Likely causes
- Compression loss in cylinder 3 from worn or stuck rings, a burnt or poorly seating valve, or a head gasket that has started to leak
- Injector for cylinder 3 under delivering because of a blocked nozzle, a worn needle or coding that does not match the installed part
- Shortfall in fuel supply to that injector, including low rail pressure, a restricted filter or air in the system
- Carbon packed into the sensor pressure channel, damping the measured peak
- Sensor that has drifted low with age and now under reports the true chamber pressure
- Valve clearance out of specification or a worn camshaft lobe reducing effective compression on that cylinder
- Intake port heavily fouled with soot, restricting the air charge reaching cylinder 3
How to fix it
- Read the freeze frame and compare live pressure traces from all cylinders at the same operating point so the shortfall can be quantified
- Inspect the sensor connector and harness, then compare the cylinder 3 signal with its neighbours at idle and at steady load
- Check rail pressure and the injector correction values for cylinder 3, and confirm the coding matches the injector fitted
- Perform a compression or leak down test on cylinder 3 and compare the results directly with the other cylinders
- If the leak down points at the valve train, check valve clearance, cam lobe wear and valve seating before suspecting the rings
- When the mechanical tests come back normal, remove the sensor and inspect the pressure channel for carbon, replacing the sensor if it reads low against a healthy cylinder
- Repair the cause found, clear the codes and confirm on a road test that the cylinder 3 peak pressure matches the rest of the engine