What does it mean?
The cylinder 3 pressure sensor is supplied with a five volt reference from the engine control module and returns a signal that should never sit at the very bottom of its range, not even with the engine stopped and the chamber at atmospheric pressure. This code is stored when the module measures a signal below the calibrated minimum for longer than the monitor allows. A healthy sensor cannot produce that reading, so the module concludes the line has found a path to ground, the reference feed has been lost, or the element inside the sensor has failed short. Without usable feedback from that cylinder the module drops injection timing and fuel quantity for cylinder 3 back to fixed calibration values. The MIL (check engine lamp) is turned on and many applications restrict engine output.
Symptoms
- MIL (check engine lamp) on with a stored fault for the cylinder 3 pressure sensor
- Rougher idle and a harsher combustion note as the module stops trimming that cylinder
- Harder cold starting and more smoke in the first seconds of running
- Power loss and, on some applications, limp mode
- Fuel consumption higher than usual because injection timing runs from a fixed map
- Slight vibration and uneven pull at low speed with light throttle
Likely causes
- Signal wire for cylinder 3 shorted to ground where the harness crosses the head, a bracket or a heat shield
- Five volt reference open or high in resistance, dragging the signal to the bottom of its range
- Sensing element shorted internally to the sensor housing
- Coolant, oil or water in the connector bridging the signal pin to the low reference
- Bent, corroded or backed out terminal at the sensor or the module connector
- Crushed section of the shared sensor harness, which normally sets codes on more than one cylinder
- Faulty module input, accepted only once the reference voltage and the wiring have both been proven correct
How to fix it
- Read the freeze frame and check whether other cylinder pressure sensors set codes at the same moment
- Inspect the connector for fluid contamination, corrosion and spread terminals, and follow the harness along the head looking for chafe and heat damage
- With the ignition on and the sensor unplugged, measure the five volt reference and the low reference at the harness side; a missing reference makes this a wiring fault
- Leave the sensor unplugged and measure resistance from the signal wire to ground, since a low reading confirms the short
- Observe the live value while wiggling the connector and harness to catch a short that only appears under movement
- Repair the harness or renew the sensor according to the measurements, using a new seal and the specified torque where the sensor is combined with the glow plug
- Clear the codes and confirm through cranking, idle and a load pull that the cylinder 3 signal now tracks the other cylinders