What does it mean?
Cylinder 4 is equipped with a combustion pressure sensor that lets the engine control module follow chamber pressure in real time, and on diesel applications the sensing element is usually housed inside the glow plug. The module provides a five volt reference and a low reference, reads the returning signal and converts it into a pressure curve referenced to crank angle, which it uses for injection timing, cylinder balancing and combustion noise control.
This code represents a general fault in that sensor circuit: the module has judged the circuit unserviceable without classifying the failure as specifically high, low or intermittent. It typically sets when the signal is missing, when the value at key on is implausible, or when the self test carried out before starting does not pass. While the code is active the module stops using cylinder 4 pressure feedback and applies a default fueling strategy.
Symptoms
- MIL (check engine lamp) on with a fault stored for the cylinder 4 pressure sensor
- Idle less refined than usual and a coarser combustion sound
- Harder cold starting and more smoke during the first seconds of running
- Loss of power because fueling for that cylinder reverts to fixed values
- Fuel consumption over a tank higher than the driver expects
- Limp mode on some applications, particularly when several cylinder pressure faults are stored together
Likely causes
- Open, short or high resistance anywhere in the cylinder 4 signal, reference or ground circuit
- Sensor connector unplugged, cracked, corroded or contaminated with oil from a leaking seal
- Failed sensing element inside the cylinder 4 sensor
- Harness chafed against the cylinder head, a bracket or a heat shield along that branch
- Damaged shared reference feed, which also disturbs the sensors on the other cylinders
- Sensor fitted loosely or with a damaged seal after earlier work, so it cannot read the chamber correctly
- Module input circuit fault, considered only after the harness and the sensor have been measured
How to fix it
- Read the freeze frame and check whether other cylinders have stored pressure sensor codes, which would point to a shared reference or ground fault
- Inspect the sensor connector for oil, coolant, corrosion, spread terminals and a damaged seal, then follow the harness looking for rub points
- With the ignition on and the connector unplugged, measure the five volt reference and the low reference on the harness side
- Check continuity of the signal wire from the sensor connector to the module pin and test it for shorts to ground and to voltage
- Reconnect and watch the live pressure value at key on and during cranking, comparing it with the other cylinders
- Wiggle the harness and connector while monitoring the live value to expose an unstable connection
- Repair the wiring or renew the sensor as indicated, refit with a new seal at the correct torque, clear the codes and verify the reading through cranking, idle and a road test