OBD-II Code P0383 — Glow Plug / Heater Circuit 'A' High

What Does This Error Mean?

P0383 means the voltage in the diesel glow plug circuit 'A' is higher than the computer expects. This is less common than a low-voltage fault but usually points to a wiring short or a problem with the control module. High voltage in this circuit can potentially damage glow plugs by overheating them. This code only applies to diesel engines. Get it diagnosed promptly to avoid damaging your glow plugs.

Affected Models

Diesel vehicles 1996+
Ford Power Stroke diesel trucks
Chevy/GMC Duramax diesel trucks
Dodge/Ram Cummins diesel trucks
European diesel vehicles (VW, BMW, Mercedes)

Common Causes

A wiring short to a voltage source in the glow plug circuit harness
A stuck-closed glow plug relay keeping voltage applied too long
A faulty glow plug control module outputting excessive voltage
Incorrect voltage from an aftermarket relay or module installed previously
A damaged glow plug connector creating an unintended voltage path

How to Fix It

Scan for all codes and note any additional codes alongside P0383. Other glow plug codes or charging system codes may help narrow down the source of the high voltage reading.

A charging system overcharging the battery can raise voltage across the entire electrical system.
Check your charging system voltage. With the engine running, measure battery voltage with a multimeter. Normal charging voltage is 13.5–14.8 volts. Anything above 15 volts indicates the alternator is overcharging and affecting all circuits.

An overcharging alternator can cause multiple high-voltage codes across the vehicle.
Inspect the glow plug relay. A relay with a stuck-closed contact will keep power flowing to the glow plug circuit even after the computer has turned it off, causing a high-voltage reading. Replace the relay if it appears to be staying on.

You can test whether the relay is stuck by unplugging it and seeing if the voltage reading drops.
Trace the glow plug circuit wiring for any points where it could contact a power source. Look for pinched wires touching the battery cable or a hot wire in the fuse box area. Repair any short to power you find.

Use a wiring diagram for your specific vehicle — these are available free on many diesel owner forums.
If the charging system and wiring check out, suspect the glow plug control module. Have a diesel mechanic test the module output voltage. A module that cannot regulate its output will need to be replaced.

Always confirm the module is faulty before buying a replacement — they are expensive at $80-$300.

When to Call a Professional

High voltage faults in the glow plug circuit are best diagnosed with a professional scan tool. A diesel specialist can monitor live voltage readings and identify which part of the circuit is out of range. Continued high voltage can burn out your glow plugs faster than normal. Expect $100-$200 for a thorough electrical diagnosis. Ask specifically for a diesel electrical technician.

Frequently Asked Questions

Can high voltage damage my glow plugs?

Yes — glow plugs are designed to operate at a specific voltage. Continuously running them at too high a voltage causes them to overheat and burn out prematurely. A glow plug that normally lasts 100,000 miles might fail in 20,000 miles if the voltage is consistently too high. Fix the root cause before replacing the plugs or they will fail again.

Is P0383 a DIY repair?

The basic checks — charging system voltage and relay inspection — are DIY friendly. But tracing a high-voltage short in the wiring harness requires a wiring diagram and some patience. If you are not comfortable with electrical work, a diesel shop can diagnose this quickly. Do not guess and replace expensive parts without confirming the fault.

How much does it cost to fix P0383?

An alternator replacement (if overcharging) runs $200-$600 installed. A glow plug relay is $15-$40. Wiring repair costs vary by the extent of damage — $50-$500. A glow plug control module is $80-$300. Get a diagnosis before spending money on parts.