What does it mean?
Circuit low means the voltage returning from the cylinder 6 combustion pressure sensor has fallen below the lowest value the control module will accept and stays there regardless of what the engine is doing. The sensor normally rests at a small bias voltage with the engine off and swings upward as the charge burns; a signal pinned near zero tells the module the line is being grounded rather than measuring pressure.
The usual electrical causes are a signal wire shorted to ground, a missing 5-volt reference, or an internal short inside the sensor itself. Because the reading is out of range low rather than merely odd, the module rejects it at once, removes cylinder 6 from closed-loop combustion control and lights the MIL. Fuelling for that cylinder then follows stored maps with no pressure feedback to correct it.
Symptoms
- MIL on, sometimes with several cylinder pressure codes together when a shared reference is involved
- Rough idle and a shake the module can no longer smooth out
- Louder, harsher combustion note, most noticeable when the engine is cold
- Power limitation or limp mode on vehicles that treat combustion feedback as critical
- Higher fuel consumption and more exhaust smoke on diesel engines
- Long cranking on diesels where the sensor forms part of the glow plug assembly
Likely causes
- Signal wire shorted to ground where the harness rubs against the cylinder head, a bracket or a heat shield
- Loss of the 5-volt reference feeding the sensor, either from an open wire or from a shorted sensor pulling the reference down
- Internally shorted or water-flooded pressure sensor
- Moisture or oil inside the sensor connector bridging the signal pin to the low reference pin
- Crushed or pinched harness section left behind after earlier work on the cylinder head or injectors
- Corroded pins forming a partial path to ground rather than a clean open circuit
- Faulty input circuit in the control module, checked last
How to fix it
- Check the freeze frame first, then look for other codes sharing the same 5-volt reference before touching the harness
- Unplug the sensor at cylinder 6 and read the signal voltage on the module side with the key on; if it rises to a normal open-circuit level the sensor is internally shorted
- If the voltage stays near zero with the sensor unplugged, the short is in the harness - test the signal wire to ground with the module connector released
- Confirm the 5-volt reference and the low reference at the sensor connector; a missing reference must be traced back before anything is replaced
- Inspect the connector for oil, coolant or moisture and dry or renew the seal as needed
- Follow the harness across the head and along any bracket for chafe marks, repairing insulation properly rather than taping over it
- Replace the pressure sensor or pressure sensing glow plug only once the wiring passes, torque it to specification, clear the code and verify a live trace