01 This Is Why Your Car’s Headrest Is A Safety Tool • 02 How to Avoid and Handle a Hydroplaning Vehicle: A Complete … • 03 Induction Motor | Working Principle, Types, Applications, M… • 04 People Are Shocked To Learn This “Normal” Driving Action Ca… • 05 Is There Any Significant Difference Vetween 1.6L and 2.0L E… • 06 You Could Destroy Your Entire Engine If You Keep Repeating … • 07 Car Security Systems: What They Are and How to Install Them • 08 Why Dirty Grounds Cause More Car Problems Than People Reali… • 09 Top 7 Reasons Your Car Loses Power: Troubleshooting Common … • 10 Maximize Fuel Efficiency: A Comprehensive Guide to Selectin… •

DTC codes / Ford codes / P1399

P1399 · FordGlow Plug Circuit High Side High Input

This code means the glow plug control module has detected an abnormally high voltage on the high side of the glow plug circuit, typically a short to battery voltage or a fault inside the module itself. The powertrain control module (PCM) sets P1399 when the feedback signal from the glow plug circuit exceeds the expected range during the self-test or while the glow plugs are commanded on.

SystemPowertrain
Code typeFord specific
SeverityMedium
Fix difficultyIntermediate
AreaIgnition system and misfire

Symptoms you may notice

  • Illuminated check engine light or glow plug warning lamp
  • Hard cold starting or extended cranking before the engine fires
  • Rough idle and white smoke immediately after a cold start
  • Often no symptoms besides the warning light, especially in mild weather
  • Possible reduced fuel economy during warm-up
  • In rare cases, a no-start condition when glow plug operation is completely disabled

Common causes, most likely first

  1. Short to battery voltage in the glow plug feed circuit (high-side wire chafed against a power source)
  2. Faulty glow plug control module internal short
  3. Damaged or corroded glow plug harness connector allowing voltage backfeed
  4. PCM software calibration issue or internal driver fault
  5. Aftermarket electrical accessory wired into the glow plug circuit
  6. Water intrusion into the glow plug module connector causing a high-voltage bridge

How to diagnose and fix it

  1. Scan all modules for related codes and record freeze-frame data to confirm when P1399 sets
  2. Visually inspect the glow plug harness and connectors for chafing, corrosion, or signs of shorting to power
  3. Measure voltage on the high-side glow plug circuit with the key on and engine off; compare to battery voltage and spec
  4. Disconnect the glow plug control module and check for voltage present on the high-side terminal with the module unplugged
  5. Test the glow plug control module according to Ford service procedure, including resistance and voltage drop checks
  6. Check for aftermarket accessories tapped into the glow plug wiring and remove or repair as needed
  7. If all wiring and components test good, reprogram the PCM with the latest calibration or replace the glow plug control module

High-side shorts that fool good glow plugs

  • A short to voltage on the high side often comes from a harness rubbed through near the battery or underhood fuse box, not from a failed glow plug itself.
  • Technicians sometimes replace all glow plugs when only the control module or a chafed wire is at fault, so always verify voltage on the high-side circuit before condemning parts.
  • Aftermarket remote starters or alarm systems tapped into the glow plug circuit can backfeed voltage and set P1399 without any factory component failure.
  • The glow plug control module can set this code intermittently when water enters its connector, so inspect for green corrosion or moisture even if the code clears.
  • P1399 can appear alongside glow plug circuit codes for individual cylinders, but the high-side fault must be repaired first or the other codes will return.

Can I keep driving with P1399?

You can usually drive the vehicle, but cold starting may be difficult and prolonged cranking can wear the starter and battery. Avoid repeated short cold-start cycles and have the circuit inspected before winter temperatures drop, as a completely failed glow plug system can leave you stranded.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Ford vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage