What does it mean?
This code is stored when the engine management module cannot obtain a usable electrical signal from the combustion chamber pressure sensor serving cylinder 7. On engines built for closed-loop combustion control, every cylinder carries a piezo pressure element - most often built into the glow plug on diesels, or into a washer under the spark plug on gasoline units - that reports the pressure trace inside the chamber cycle by cycle.
The module feeds that element a regulated 5-volt reference and a low reference, then reads the returning signal to work out peak pressure, the crank angle at which the charge actually burns and the work the cylinder produces. A general circuit fault means the signal line is open, shorted or otherwise outside any plausible electrical window, so there is no pressure trace to work with and cylinder 7 drops out of the combustion feedback loop.
Fuelling and timing then fall back to stored map values, which is why the fault lights the MIL rather than being tolerated quietly.
Symptoms
- MIL on, often accompanied by pressure sensor codes for neighbouring cylinders
- Rough, uneven idle because the module can no longer trim that cylinder individually
- Noticeably harsher combustion noise, most obvious on a cold engine
- Reduced power and slower throttle response as the module reverts to conservative base maps
- Higher fuel consumption and, on diesels, more visible smoke under load
- Harder cold starting on diesel engines where the sensor shares its housing with the glow plug
Likely causes
- Corroded, spread or water-damaged terminals in the pressure sensor connector at cylinder 7
- Open circuit in the signal, 5-volt reference or low reference wire, typically where the harness runs close to the cylinder head and the insulation has hardened with heat
- Signal wire shorted to ground or to battery voltage through chafed insulation
- Internal failure of the pressure sensing element, common once the sensor has seen high mileage and repeated heat cycling
- Loss of the shared 5-volt reference feeding several sensors, which usually sets codes on more than one cylinder at once
- Poor engine-to-body or module ground, distorting the low reference the sensor is measured against
- Failed input stage inside the engine management module, considered only after the wiring has been proven good
How to fix it
- Read the freeze frame and note whether the fault appeared cold, warm or under load, then check for companion codes pointing at a shared reference or ground
- Inspect the sensor connector and the harness section above cylinder 7 for melted insulation, chafe points and green or white corrosion on the pins
- With the key on and the connector unplugged, confirm roughly 5 volts on the reference wire and near zero on the low reference; a missing reference sends you upstream toward the module
- Measure signal wire continuity and resistance to the module pin, then test for shorts to ground and to battery with the module connector released
- Wiggle-test the harness while graphing the live pressure signal so an intermittent break shows itself before parts are ordered
- Compare the live pressure trace of cylinder 7 against a known-good cylinder; a flat or absent trace with good wiring confirms the sensor
- Fit the replacement sensor or pressure sensing glow plug at the specified torque, since seating force directly affects signal accuracy, then clear the code and verify the trace under load