P1399 indicates that the engine control module has detected an excessive voltage on the high‑side side of the glow‑plug circuit, meaning the voltage is higher than the calibrated limit.
The internal driver that switches the high‑side voltage can fail shorted, allowing the supply voltage to appear at the glow‑plug circuit.
A short to power or a damaged insulation on the high‑side wiring can raise the voltage seen by the control module.
One or more glow plugs that have failed internally can feed voltage back into the circuit, causing a high‑side reading.
A regulator that allows excessive battery voltage to reach the glow‑plug circuit can trigger the code.
Only after ruling out the hardware above should a rare ECM sensor or processing fault be considered.
No. The PCM is rarely the cause of a high‑side voltage fault. Replacing it without confirming a hardware failure is usually unnecessary and costly.
A defective glow plug can contribute, but it must have an internal short that feeds voltage back into the circuit. Many times the fault is in the control module or wiring instead.
Yes. P1399 pertains to the glow‑plug system, which is used on diesel powerplants.
The code will clear after the PCM completes its drive‑cycle checks without detecting a fault. If it remains, a manual scan tool reset may be required.