P0671: cylinder 1 glow-plug open-load diagnosis

P0671 identifies an open-load condition at cylinder 1's glow output in the cited hardware monitor. It assesses the connected circuit's electrical integrity rather than directly measuring the plug's heating rate.

Two resistance statements define the monitor

The source specifies detection with a 200-kilohm load condition and says an open circuit should not be detected below 16 ohms. It does not define the entire intermediate region as passing or failing, and neither value is a complete normal glow-plug resistance specification.

This distinction matters when reviewing a meter reading: the table's detection boundaries do not replace the exact vehicle's component-test limits. An open-load code also cannot locate the discontinuity within the plug, a connection or the driver path by itself.

Confirm active monitoring

The engine must be running outside cranking, battery voltage valid, glow-system supply free of the listed fault and diagnostics enabled. Duty cycle must be above 2.50% and below 97.50%. The filter uses ten failed samples among twenty on a 100 ms task, with Type B reporting and glow-lamp indication.

Verification should establish that the cylinder-1 circuit is electrically loaded during a valid monitoring period. A check performed with a supply fault or outside the duty window is inconclusive. Use the approved service method; the summary does not prescribe direct battery testing, live resistance measurement or glow-plug removal.

Source and applicability

2025 GM 25OBDG04B diagnostic parameter tables; the cited monitor only. Vehicle, engine and calibration applicability must be confirmed separately. These tables describe detection logic, not a complete repair procedure or a component replacement verdict. gm-2025-04b, PDF page 1238.