OBD-II Code P0389 — Glow Plug / Heater Circuit 'B' Open

What Does This Error Mean?

P0389 means the 'B' glow plug circuit on your diesel engine has an open circuit — electricity is not flowing through it at all. An open circuit is usually caused by a broken wire, a blown fuse, a failed relay, or a burned-out glow plug that has snapped internally. The 'B' circuit covers one bank of cylinders on a V-engine diesel. All of the glow plugs on that bank are getting no power. Expect very difficult cold starts in any cold weather.

Affected Models

V6 and V8 diesel vehicles 1996+
Ford Power Stroke V8 diesel trucks
Chevy/GMC Duramax V8 diesel trucks
Mercedes diesel V6 and V8 engines
Volkswagen and Audi diesel engines

Common Causes

A completely failed or burned-out glow plug with an open internal element
A blown fuse in the glow plug circuit for the 'B' bank
A failed glow plug relay that is no longer passing current
A broken wire in the 'B' circuit harness with no electrical continuity
A failed glow plug control module that no longer activates the 'B' circuit output

How to Fix It

Check the fuse for the glow plug circuit. Consult your vehicle's fuse box diagram to find the correct fuse. A blown fuse is the easiest and cheapest fix — replace it with the correct amperage fuse and see if the code clears.

If the fuse blows again immediately, there is a short circuit in the system that must be found before replacing the fuse again.
Test the glow plug relay for the 'B' circuit. Remove it and test with a multimeter for continuity when activated. A relay with no continuity is an open circuit failure — replace it.

A relay is $15-$40 and is one of the most common open-circuit causes.
Check each glow plug on the 'B' bank for continuity with a multimeter set to resistance. A plug reading infinity (OL) has an open internal element — it is dead and must be replaced. Even one open plug can cause this code.

If multiple plugs are open, the wiring may be the issue rather than the plugs themselves.
Trace the 'B' circuit wiring harness from the relay to each glow plug. Use a multimeter to check continuity along the harness. A section with no continuity has a broken wire that must be repaired or replaced.

Pay attention to any point where the harness bends sharply, runs through a grommet, or passes near a hot component.
If fuse, relay, plugs, and wiring all have continuity, the glow plug control module may not be closing the circuit on the 'B' side. A scan tool that can command glow plug activation and measure circuit response will confirm this.

Module failure on just one circuit output is possible — a diesel specialist with the right scan tool can verify this in minutes.

When to Call a Professional

An open circuit means zero current flow — the 'B' bank glow plugs are completely inactive. This is a definitive fault that will cause hard cold starts immediately in cold weather. If you are not comfortable with electrical testing, a diesel shop can diagnose and repair this quickly. Expect $100-$200 for the diagnostic work and separate costs for any parts.

Frequently Asked Questions

What does 'open circuit' mean in plain English?

Think of it like a light switch. When the switch is on, electricity flows and the light works. An open circuit is like a broken wire in the middle of that path — the switch is on, but the electricity has nowhere to go. In the glow plug circuit, an open means the plugs on the 'B' bank receive no power at all, so they cannot heat up.

How serious is P0389 in cold weather?

Very serious. Diesel engines rely on glow plugs to pre-heat the combustion chamber in cold weather. With the entire 'B' bank inactive, half your glow plugs are not working. Below freezing temperatures, your diesel may refuse to start, or you will need to crank it for an extended time and deal with heavy white smoke. Do not ignore this going into winter.

How much does it cost to fix P0389?

A blown fuse: under $5. A glow plug relay: $15-$40. Individual glow plugs: $10-$30 each. Full bank replacement with labor: $150-$400. Wiring harness repair: $50-$500 depending on damage. Glow plug control module: $80-$300.