Glow Plug Circuit High Side
The glow plug circuit's high-voltage control line is stuck too high or has excessive voltage, preventing proper glow plug operation. Think of it like a light switch that's jammed in the 'on' position—the system can't turn the heating element on and off correctly.
The ECU monitors the voltage and current flowing through the glow plug relay control circuit. It expects voltage to drop to near zero when the relay is off and rise to battery voltage when activated. A high-side fault occurs when voltage remains elevated above normal cutoff thresholds, indicating a stuck relay, wiring short, or open return path.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Glow plug control voltage | 0V (off) to 12-14V (on), cycling as needed | Sustained voltage above 10V or stuck at battery voltage |
| Glow plug circuit current | 0-50A peak, cycling pattern | Continuous high current draw or no current modulation |
Code P1404 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P1404 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.