OBD-II Code P0380 — Glow Plug / Heater Circuit 'A' Malfunction

What Does This Error Mean?

P0380 means there is a fault in the glow plug or heater circuit labeled 'A' on your diesel engine. Glow plugs are what heat the combustion chamber before you start a cold diesel engine. Without working glow plugs, your diesel will struggle to start — especially on cold mornings. This code does not affect gasoline engines. Get this checked before winter arrives or hard starts will leave you stranded.

Affected Models

Diesel vehicles 1996+
Ford Power Stroke diesel trucks
Chevy/GMC Duramax diesel trucks
Dodge/Ram Cummins diesel trucks
VW TDI diesel cars

Common Causes

One or more failed glow plugs that no longer heat properly
A broken or corroded wire in the glow plug circuit harness
A failed glow plug relay that controls power to the plugs
A faulty glow plug control module that manages timing and voltage
High resistance in the glow plug circuit due to age or corrosion

How to Fix It

Read all stored codes with an OBD-II scanner. Write down every code present. P0380 is often accompanied by other glow plug codes like P0381, P0382, or P0383 that identify which specific plug or circuit is affected.

A scanner with live data can show you the glow plug relay activation signal in real time.
Locate and test the glow plug relay. On most diesel trucks it is in the under-hood fuse/relay box. Pull the relay out and swap it with a matching relay from another slot to test it, or buy a new relay — they typically cost $15-$40.

A failed relay is the easiest and cheapest fix — always start here before touching the plugs themselves.
Visually inspect the wiring harness running to each glow plug. Look for melted insulation, broken connectors, or green corrosion on the terminals. Repair or replace any damaged wiring.

Glow plug wires get extremely hot and are prone to heat damage over time.
Test each glow plug with a multimeter set to resistance (ohms). A good glow plug reads 0.5–2 ohms. A reading of infinity (open circuit) means that plug is dead and must be replaced.

Replace all glow plugs as a set if the engine has high mileage — they wear out together.
If the relay, wiring, and plugs all test good, the glow plug control module itself may be faulty. This is a more expensive part — confirm with a diesel mechanic before replacing it.

Glow plug control modules can cost $80-$300 depending on the vehicle.

When to Call a Professional

If you have replaced the glow plug relay and still get P0380, take it to a diesel specialist. Testing individual glow plugs requires a multimeter and some mechanical skill. A broken glow plug that snaps off inside the head is a serious repair costing $500-$2,000. A diesel shop can test resistance on each plug quickly and safely. Expect $80-$150 for a diagnosis.

Frequently Asked Questions

Will P0380 prevent my diesel from starting?

In mild weather, probably not — the engine may still start with reduced performance. In cold weather (below 32°F / 0°C), yes — bad glow plugs can make a cold start very difficult or impossible. The colder it is, the more you need working glow plugs. Don't ignore this code heading into winter.

How many glow plugs does my engine have?

One per cylinder. A 4-cylinder diesel has 4 glow plugs. A 6-cylinder diesel (like a Cummins or Duramax) has 6. A diesel V8 (like Power Stroke) has 8. You can replace just the bad one, but if the engine has over 100,000 miles, replacing all of them at once saves labor costs later.

How much does it cost to fix P0380?

A glow plug relay costs $15-$40 and is a DIY fix. Individual glow plugs cost $10-$30 each. A full set of glow plugs with labor runs $150-$600 depending on how many cylinders. If a plug breaks off during removal, costs can jump to $500-$2,000 for extraction and repair.