What does it mean?
The module verifies not only that the cylinder 4 pressure signal is electrically alive but also that the value it reports is credible for the conditions the engine is in at that instant. With the engine stopped the sensor should read close to atmospheric pressure, during compression it should rise in a known relationship to crank angle, and during combustion the peak should land inside a predictable band.
This code is stored when the signal stays within its electrical limits yet fails one of those plausibility checks, so the voltage is legal while the pressure it represents is not. A shifted zero point, a damped or blocked pressure channel and a genuine mechanical change in the cylinder can all lead to it. Since the signal alone cannot separate a lying sensor from a sick cylinder, the module disables closed loop combustion control for cylinder 4 and turns on the MIL (check engine lamp).
Symptoms
- MIL (check engine lamp) on, sometimes without any dramatic change in the way the vehicle drives
- Idle slightly less smooth and a combustion note the driver may call harsher
- Cold starting takes a little longer and produces more smoke than normal
- Mild power loss and a small but consistent rise in fuel consumption
- Cylinder balance or combustion performance codes appearing over time
- Occasional unevenness under light load that varies with engine temperature
Likely causes
- Sensor whose zero point or gain has drifted with age, so the reported pressure no longer matches reality
- Carbon restricting the pressure channel in front of the sensing element and distorting the curve
- Sensor not seated correctly, tightened to the wrong torque or fitted with a damaged sealing washer
- Wrong sensor variant installed, with an output characteristic the calibration does not expect
- High resistance in the signal, reference or ground circuit shifting the whole reading by a fixed offset
- Genuine mechanical change in cylinder 4 such as low compression or a leaking valve that moves the trace outside the model
- Valve timing or valve train problem that shifts the pressure peak away from the expected crank angle
How to fix it
- Compare the live pressure readings from all cylinders with the ignition on and the engine stopped; they should all sit close together
- Check the compression and injector correction values for cylinder 4 to establish whether the cylinder itself is healthy
- Inspect the connector and measure the resistance of the signal, reference and ground circuits back to the module
- Remove the sensor and examine the tip, the pressure channel and the sealing washer for carbon packing and damage
- Verify the part number of the fitted sensor against the specification, since a substituted variant reads with the wrong characteristic
- Carry out a compression or leak down test on cylinder 4 if both the sensor and its circuit check out
- Refit or renew the sensor at the specified torque with a new seal, clear the codes and confirm the trace now matches the other cylinders at idle and under load