What does it mean?
Cylinder 1 carries a pressure sensor that reports chamber pressure to the engine control module, and on diesel applications it is very often integrated into the glow plug so that the diaphragm faces the combustion chamber directly. The module powers the sensor with a five volt reference, returns it through a low reference and reads a signal voltage that should stay well inside a defined window at all times, including with the engine stopped.
This code is stored when that signal voltage remains above the calibrated maximum for longer than the monitor allows. A signal pinned at the top of its range indicates that the return path to the module has been lost, that the line has picked up a higher voltage, or that the element inside the sensor has failed open. Because the module can no longer trust the combustion feedback from cylinder 1, it switches that cylinder to a default fueling strategy.
Symptoms
- MIL (check engine lamp) on with a fault stored against the cylinder 1 pressure sensor
- Idle quality drops and the engine sounds harsher than usual
- More difficult starting when cold and heavier smoke on the first start of the day
- Noticeable power loss because combustion feedback for that cylinder is discarded
- Increased fuel consumption and, on some applications, limp mode
- A slight vibration or uneven pull under light acceleration that the driver can feel
Likely causes
- Open or high resistance low reference circuit, which allows the signal line to float to reference voltage
- Signal wire shorted to the five volt reference or to battery voltage inside a damaged harness
- Broken signal conductor so that only the module internal pull up is seen at the input
- Internally open sensing element or a fractured connection inside the sensor body
- Corroded terminals adding enough series resistance to push the reading to the top of the range
- Water or salt in the connector creating a leakage path from an adjacent powered pin
- Module input fault, considered last and only after the harness measurements are proven good
How to fix it
- Read the freeze frame and check whether pressure sensor codes exist on other cylinders, which would move the diagnosis to a shared circuit
- Inspect the connector for corrosion, water, damaged seals and pushed back terminals, and check the harness for chafing where it runs over the head
- With the ignition on and the sensor connected, backprobe the signal wire; a value locked near reference voltage confirms the condition is present
- Unplug the sensor and measure the low reference to ground at the harness side, since high resistance there is the most frequent cause
- Confirm continuity of the signal wire back to the module and check that it is not shorted to the five volt reference or to a powered circuit
- Compare the suspect sensor with a known good unit of the same specification if one is available, watching whether the fault follows the sensor
- Repair or renew the faulty item, clear the memory and confirm on a full warm up and road test that the cylinder 1 pressure signal now sits within range