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GTECH Calibration
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P1399

Glow Plug Circuit High Side High Input.

  • Mercury
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
3
AreaIgnition system or misfire
99
FaultThe specific fault in that area

What it means

P1399 on a Mercury means the Glow Plug Circuit High Side is reading a high input, as it does on Ford applications. The powertrain control module monitors the voltage on the glow plug control circuit and saw a level higher than expected for the commanded state. This points to a fault in the glow plug feed, relay or control circuit rather than a problem elsewhere on the vehicle.

Symptoms

  • Hard starting or extended cranking when the engine is cold
  • White smoke and rough running after a cold start
  • Glow plug or wait-to-start warning lamp behaving abnormally or staying on
  • Engine management warning lamp illuminated
  • Noticeably worse cold-weather starting than usual

Possible causes

  • Glow plug relay or control module stuck on or failing internally, giving a high voltage on the high side circuitlikely
  • Short to voltage in the glow plug feed or control wiringlikely
  • Corroded, loose or damaged connector at the glow plug relay, control module or glow plug bus barpossible
  • Faulty glow plug or plugs causing an abnormal circuit readingpossible
  • Charging or battery voltage problem giving a high supply voltage that the module reads as a faultpossible
  • Fault inside the powertrain control modulerare

How to diagnose

  1. Read the stored codes and freeze frame data, noting engine temperature, battery voltage and whether other codes are present
  2. Check battery condition and charging voltage to rule out a supply voltage issue
  3. Visually inspect the glow plug relay, fuses, wiring and connectors for heat damage, chafing, corrosion and water ingress
  4. Check the glow plug circuit for a short to voltage with the ignition off, following the wiring diagram for the specific model and engine
  5. Test the glow plug relay or control module operation and check it switches off when it should
  6. Measure the resistance of each glow plug and check the bus bar or feed connections
  7. Monitor the glow plug command and circuit voltage with a scan tool during a cold and warm key-on
  8. Clear the code and road test to confirm it does not return after the repair

How to fix

  • Replace a faulty glow plug relay or glow plug control module
  • Repair or replace shorted, chafed or damaged wiring in the glow plug circuit
  • Clean or replace corroded or damaged connectors and terminals
  • Replace any glow plugs that are out of specification
  • Correct battery or charging system faults
  • Replace or reprogram the powertrain control module only after all other causes are ruled out and as the manufacturer specifies

Can I keep driving?

Medium severity

The car can usually be driven short term, but cold starting may be poor and a stuck-on circuit could overheat parts or damage glow plugs. Get it checked soon.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

This code for other makes

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