P0671
Cylinder 1 Glow Plug Circuit
If your vehicle's onboard computer has flagged the diagnostic trouble code P0671, it refers to a detected anomaly regarding "Cylinder 1 Glow Plug Circuit". This systemic engine fault needs a targeted check before symptoms expand.
Driver's Summary
Storing code P0671 is your car's way of telling you something is wrong with the cylinder 1 glow plug circuit. In practice, this fault causes extended crank time when cold, rough idle on startup, check engine light. No immediate danger, but addressing this soon will prevent potential emissions test failures and minor system degradation.
Symptoms
Extended crank time when cold, rough idle on startup, check engine light
Common Causes
- Defective glow plug on cylinder 1
- Damaged glow plug wiring or connector
- Failed glow plug control module
- Corroded electrical connection
How to Fix
- 1 Replace cylinder 1 glow plug
- 2 Repair or replace glow plug harness
- 3 Clean connector pins
- 4 Test control module output
Technical Explanation
The ECM detects code P0671 by continuously monitoring the relevant sensor circuit against calibrated threshold values stored in its non-volatile memory. 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. The MIL illuminates after the fault is confirmed on two consecutive drive cycles, and the freeze frame data captured at first detection is stored in the PCM's memory for diagnostic reference.
Is It Safe to Drive?
This code won't strand you, but it shouldn't be ignored indefinitely. The defective glow plug on cylinder 1 issue identified by P0671 can mask other developing problems and will cause an automatic emissions test failure in most states.
Mechanic's Pro Tip
Module replacement should always be the last resort for P0671 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.
Single glow plug: $80 - $200