P0674
Cylinder 4 Glow Plug Circuit
Encountering the engine check light code P0674 signifies an explicit mechanical or electrical operational breakdown categorized as "Cylinder 4 Glow Plug Circuit". Professional scanner tools usually flag this subsystem loop early on.
Driver's Summary
A P0674 fault code points directly to a problem with cylinder 4 glow plug circuit that the ECM has confirmed over multiple drive cycles. On the road, this usually shows up as noticeable diesel knock on cold start, mil illuminated. This is a low-urgency fault with minimal immediate impact on safety, but it should be resolved before your next emissions test.
Symptoms
Noticeable diesel knock on cold start, MIL illuminated
Common Causes
- Defective cylinder 4 glow plug
- Wiring harness shorted to engine block
- Failed control module
- Damaged electrical connector
How to Fix
- 1 Replace glow plug
- 2 Inspect and repair wiring harness
- 3 Test module output voltage
- 4 Replace connector boot
Technical Explanation
The PCM triggers P0674 after its internal monitoring routine detects that a specific circuit or sensor has exceeded its acceptable operating range. The control module samples the circuit continuously during normal operation, using both voltage level monitoring and frequency analysis to detect open circuits, shorts to ground, shorts to battery voltage, and high-resistance connections. After two failed drive cycles, the code transitions from a pending to a confirmed DTC, and the PCM activates the MIL. Clearing the code without repairing the fault will result in re-illumination within one to two complete drive cycles.
Is It Safe to Drive?
Immediate safety risk is low with P0674 active. The primary concern is regulatory — this fault will cause a failed emissions test — and the secondary risk is that the small root cause (defective cylinder 4 glow plug) becomes a larger problem if ignored for months.
Mechanic's Pro Tip
Module replacement should always be the last resort for P0674 after exhaustively verifying all power supply circuits, ground connections, and communication bus wiring. Use a wiring diagram to locate all fuses, relays, and ground points for the affected module, and measure voltage drop on each ground with the circuit loaded. A module "failure" is frequently a corroded ground eyelet or a weak battery causing brownout conditions — fix these first and you'll save hundreds of dollars on an unnecessary module replacement.
Glow plug: $80 - $200