What does it mean?
Cylinder 3 carries a combustion pressure sensor that reports chamber pressure to the engine control module in real time, and on diesel applications it is normally built into the glow plug so the diaphragm faces the combustion chamber directly. The module supplies a five volt reference and a low reference, reads the returning signal and converts it into a pressure curve tied to crank angle for injection timing, cylinder balancing and combustion noise control.
This code is a general circuit fault: the module has judged the circuit faulty without classifying the failure specifically as high, low or intermittent. It usually sets when the signal is absent, when the reading at key on is implausible, or when the self test performed before the engine starts does not pass. While the code is active the module stops using cylinder 3 pressure feedback and applies a default fueling strategy.
Symptoms
- MIL (check engine lamp) on with a fault stored for the cylinder 3 pressure sensor
- Idle that feels 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 present together
Likely causes
- Open, short or high resistance anywhere in the cylinder 3 signal, reference or ground circuit
- Sensor connector unplugged, cracked, corroded or contaminated with oil from a leaking seal
- Failed sensing element inside the cylinder 3 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 neighbouring 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 points 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 while 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