P1399
FordGlow Plug Circuit High Side, High Input
The engine computer detected a higher-than-expected electrical input in the glow plug circuit's high-side control. Glow plugs warm a diesel engine's cylinders to help with cold starting. This fault may affect cold starts, but the code alone does not identify a failed part.
⚠️ Before You Begin (Safety First)
A glow plug circuit fault can make a diesel harder to start, especially when the engine is cold. If the engine starts and runs normally, the code alone does not usually mean you must stop driving. Stop if the engine stalls, runs severely poorly, or another warning indicates an urgent problem. Keep clear of hot engine parts, and do not probe energized circuits unless you know the correct safe procedure.
Common Symptoms
- The check-engine light may be on because the engine computer detected the circuit condition.
- Cold starting may take longer or become difficult if the glow plugs are not being controlled or powered as intended.
- The engine may run unevenly briefly after a cold start if cylinders are not warmed adequately.
- There may be no noticeable symptoms, particularly in warm conditions or if the circuit fault is intermittent.
Common Causes
- Damaged, chafed or pinched wiring may alter the high-side circuit input.
- A loose, corroded or damaged connector may cause an abnormal electrical reading.
- A glow plug or related circuit component may have an electrical fault.
- A fault in the glow plug control circuit or module may produce the detected high input.
- An intermittent connection or wiring fault may cause the condition to appear only at certain times.
Diagnostic Steps
- Record the code and note whether the starting problem occurs when the engine is cold; this helps establish whether the circuit fault is affecting cold-start operation.
- With the engine off and cool, visually inspect accessible glow plug circuit wiring and connectors for damage, looseness or corrosion.
- Do not unplug or test energized components unless the vehicle-specific service information explains a safe procedure.
- If the code returns or wiring damage is not obvious, have the circuit tested using the correct wiring diagram and model-specific diagnostic information.
When to Call a Professional
Confirm the code and check for related codes before replacing parts. Use the vehicle's model-, year- and engine-specific wiring information to inspect the glow plug high-side circuit, connectors, wiring, power supply and control module. Electrical testing should determine whether the high reading is caused by a wiring or connection problem, a control issue, or a glow plug circuit component.
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Commonly Asked Questions
Does P1399 prove that a glow plug needs replacement?
No. It identifies a high input in the glow plug circuit's high side, not a confirmed failed part. Wiring, connectors, control components and the glow plug circuit itself need to be checked.
Can I drive with this code?
If the engine starts and runs normally, the code alone does not usually require you to stop immediately. Cold starting may be affected. Stop if the engine stalls or runs severely poorly, and address the fault promptly if starting becomes unreliable.
Why does the problem show up more during cold starts?
Glow plugs warm the cylinders to help a diesel engine start when cold. A circuit fault may prevent them from operating as intended, while warmer conditions can make the effect less noticeable.
Could a wiring problem cause a high input?
Yes. Damaged wiring, a poor connection or a control-circuit fault can affect the monitored input. The circuit should be tested before replacing a glow plug or module.
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