Glow Plug Circuit Low Input (Bank #2)
The glow plug heating system on the diesel engine's second cylinder bank isn't getting enough electrical power to warm up properly. It's like a heating element that's receiving only a weak current instead of the strong signal it needs.
The ECU monitors the voltage and current supplied to the glow plugs in Bank 2 through a dedicated relay circuit. It expects a specific voltage level during the glow plug preheat phase before combustion begins. If voltage drops below the minimum threshold, the ECU detects a low-input fault.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Glow Plug Circuit Voltage | 10-14 volts during preheat cycle | Below 8 volts or open circuit detected |
| Glow Plug Relay Current Draw | 8-15 amps under load | Less than 2 amps or excessive resistance |
Code P1398 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, P1398 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.