What does it mean?
The combustion pressure sensor for cylinder 1 measures what actually happens inside that cylinder during compression and burn, and on most diesel applications it is built into the glow plug body so the sensing element sits directly in the chamber. The engine control module supplies the sensor with a five volt reference and a low reference, then reads the returning signal and converts it into a pressure curve referenced to crank angle.
That curve is used to set injection timing, trim fueling cylinder by cylinder and keep combustion noise and emissions within limits. This code is stored when the signal line stays below the calibrated minimum voltage for longer than the monitor allows, a level the sensor should never produce even with the engine stopped. A signal pinned near zero volts tells the module the line is grounded, the reference feed has been lost or the sensing element has failed short.
Symptoms
- MIL (check engine lamp) on with a stored fault naming the cylinder 1 pressure sensor
- Rougher idle and a coarser combustion note as the module loses the feedback used to trim that cylinder
- Harder cold starting and more smoke during the first seconds of running
- Reduced power because fueling reverts to a fixed default map, and limp mode on some applications
- Higher fuel consumption and, on diesel applications, more combustion noise
- Uneven running that the driver notices most at low speed and light throttle
Likely causes
- Short to ground in the cylinder 1 signal wire, most often where the harness passes over the cylinder head or a bracket
- Open or high resistance in the five volt reference feed, which drags the signal to the bottom of its range
- Sensing element that has failed internally and shorted to its own housing
- Oil, coolant or water in the sensor connector bridging the signal pin to the low reference
- Corroded, bent or backed out terminal at the sensor or at the module connector
- Damage to the shared harness section that serves all cylinder pressure sensors, which normally sets codes on the neighbouring cylinders as well
- Failed module input, accepted only after the wiring and the reference voltages have been proven correct
How to fix it
- Record the freeze frame and check whether the other cylinder pressure sensors have set codes too, since that points to a shared reference or ground problem rather than one sensor
- Inspect the sensor connector for oil, coolant, corrosion and spread terminals, and follow the harness along the head looking for chafing and heat damage
- With the ignition on and the sensor unplugged, measure the five volt reference and the low reference on the harness side; a missing reference is a wiring fault, not a sensor fault
- With the sensor still unplugged, measure resistance from the signal wire to ground; any low reading confirms the short
- Watch the live pressure value while wiggling the harness and the connector to expose a partial short that only appears under movement
- Repair the harness or renew the sensor as the measurements dictate, observing the correct torque and sealing if the sensor is combined with the glow plug
- Clear the codes and confirm during cranking, idle and a load pull that the cylinder 1 pressure trace now follows the other cylinders