OBD-II Code P0387 — Glow Plug / Heater Circuit 'B' High

What Does This Error Mean?

P0387 means the voltage on the 'B' glow plug circuit of your diesel engine is higher than the computer expects. This is a less common fault than a low-voltage code, but it can cause glow plugs to burn out prematurely. The 'B' circuit covers one bank of cylinders on a V-configuration diesel engine. The most common causes are a stuck relay, a wiring short to voltage, or an overcharging alternator. This code only applies to diesel engines.

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
Land Rover and Range Rover diesel models

Common Causes

A stuck-closed glow plug relay for the 'B' circuit keeping voltage applied continuously
A wiring short to a power source in the 'B' circuit harness
An overcharging alternator raising voltage across the entire electrical system
A faulty glow plug control module sending excess voltage to the 'B' circuit
An aftermarket relay or module wired incorrectly causing unregulated voltage

How to Fix It

Check the charging system voltage with a multimeter while the engine runs. Normal is 13.5–14.8 volts at the battery. If you read 15 volts or more, the alternator is overcharging and raising voltage across all circuits, which can trigger P0387.

An overcharging alternator also causes other electrical issues — lights flickering, battery damage, and other voltage codes.
Test the glow plug relay for circuit 'B'. A relay with stuck-closed contacts will keep voltage applied to the glow plugs even after the computer has commanded them off. Replace the relay if you suspect it is not releasing correctly.

You can check if the relay is stuck by unplugging it when the engine is warm — glow plugs should not be active after warm-up.
Inspect the wiring harness for the 'B' bank glow plugs. Look for any wire that may be contacting a hot wire or a battery terminal. A short to voltage will hold the circuit high regardless of what the control module commands.

Use a wiring diagram to identify which wires belong to the glow plug circuit.
If all external checks pass, have the glow plug control module tested. A module with a failed internal regulator can send excessive voltage to just one circuit while the other operates normally.

A scan tool with bi-directional controls can command the glow plugs on and off to verify the module is responding correctly.
After repairing the fault, test each glow plug to make sure none were burned out by the high voltage. Replace any plug outside the 0.5–2 ohm resistance range.

Sustained high voltage can burn through glow plugs quickly — always check them after a high-voltage event.

When to Call a Professional

High voltage faults in electrical circuits are best confirmed with a professional-grade scan tool and wiring diagrams. If the charging system is fine and the relay has been replaced, a diesel electrician can trace the circuit systematically. Continued exposure to high voltage will shorten glow plug lifespan considerably. Expect $100-$200 for electrical diagnosis.

Frequently Asked Questions

Will P0387 damage my glow plugs?

Yes, over time. Glow plugs are designed for a specific voltage range. Running them too hot burns out the heating element faster. A plug that would normally last 100,000 miles might fail in 20,000-30,000 miles under sustained high voltage. Fix the root cause first, then inspect all plugs on the 'B' bank.

How do I know if my alternator is overcharging?

Use a basic multimeter and measure the voltage directly at the battery with the engine running. Normal is 13.5 to 14.8 volts. Anything above 15 volts is overcharging. You might also notice headlights that are unusually bright or LED lights flickering. An overcharging alternator will eventually destroy the battery as well.

How much does it cost to fix P0387?

Alternator replacement: $200-$600 installed. Glow plug relay: $15-$40. Wiring repair: $50-$400 depending on extent of damage. Glow plug control module: $80-$300. Replacing burned-out glow plugs: $10-$30 each, plus $100-$250 in labor.