What does it mean?
Once the module has a trustworthy pressure trace from cylinder 1 it evaluates the peak value reached during the power stroke and compares it with what the calibration expects for the current speed, load, boost and fuel quantity. This code is stored when that measured peak stays below the lower limit, so the cylinder is producing less pressure than the model says it should.
Unlike the circuit codes in the same family, the electrical signal here is valid and inside its normal range, which pushes the diagnosis toward mechanical condition, fuel delivery or a sensor that has drifted low. A cylinder that cannot build pressure contributes less torque than its neighbours, so the module usually also detects a contribution imbalance. The MIL (check engine lamp) is turned on and, on many applications, power is limited to protect the engine.
Symptoms
- MIL (check engine lamp) on, often with contribution, balance or misfire codes for the same cylinder
- Rough idle with a repeating beat that the driver can hear and feel
- Poor starting when cold and grey or white smoke during the first minute of running
- Clear loss of power and slow response when accelerating with a load
- Higher fuel consumption as the module tries to compensate for the weak cylinder
- Vibration through the body at low engine speed and, on some applications, limp mode
Likely causes
- Loss of compression in cylinder 1 from worn or stuck piston rings, a burnt or poorly seating valve, or a leaking head gasket
- Injector for cylinder 1 delivering too little fuel because of a blocked nozzle, a worn needle or incorrect coding after replacement
- Injection timing retarded on that cylinder as a result of a fuel supply problem or a low rail pressure condition
- Carbon or soot blocking the pressure channel in front of the sensing element, damping the reading
- Sensor that has drifted low with age and now under reports true chamber pressure
- Air entering the fuel system through a leaking supply line or a failing filter housing seal
- Valve clearance out of specification or a worn camshaft lobe reducing effective compression on that cylinder
How to fix it
- Read the freeze frame and compare the live pressure traces of all cylinders at idle so the size of the deficit is known
- Inspect the sensor and its connector for contamination and check the harness, then compare the cylinder 1 signal with the neighbouring cylinders at the same operating point
- Check fuel rail pressure and the correction values for the cylinder 1 injector, and confirm the injector coding matches the part fitted
- Carry out a compression test or a cylinder leak down test on cylinder 1 and compare the results directly with the other cylinders
- If leak down points to the valve train, check valve clearance, cam lobe wear and valve seating before assuming ring wear
- Inspect the sensor pressure channel for carbon blockage when the mechanical tests are normal, and renew the sensor if it under reports against a healthy cylinder
- Repair the cause found, clear the codes and verify with a road test that the cylinder 1 peak pressure now matches the other cylinders