The cylinder 4 glow plug circuit delivers battery power to the rearmost cylinder's heating element. The wiring runs through the main engine harness. If this harness is pinched against the firewall, it can break the power wire or cause a short. The engine control module monitors circuit resistance. It triggers a trouble code if the connection is lost.
Mechanic Glossary
4 Glow Plug Circuit
The electrical harness and power line routing current from the preheat relay to cylinder 4 glow plug.
Frequently Asked Questions
A broken wire, a loose harness plug, or corrosion at the cylinder 4 terminal.
Check for 12 volts at the cylinder 4 connector using a test light or multimeter while cycling the ignition key to the run position.
In cold weather, yes. The cylinder 4 glow plug won't heat up, causing rough starts and engine misfires until it warms up.