OBD-II Code P0386 — Glow Plug / Heater Circuit 'B' Low

What Does This Error Mean?

P0386 means the voltage on the 'B' glow plug circuit of your diesel engine is lower than expected. The 'B' circuit covers one bank of cylinders — typically the passenger side on a V-engine. Low voltage in this circuit means the glow plugs on that bank are not heating up fully before cold starts. The most likely causes are a shorted glow plug, a failing relay, or damaged wiring. 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
Dodge/Ram Cummins diesel trucks

Common Causes

A shorted glow plug on the 'B' bank pulling the circuit voltage down
A weak or failing glow plug relay for the 'B' circuit
Corroded or loose wiring connections creating resistance and dropping voltage
A discharged or weak battery unable to supply full voltage to the glow circuit
A faulty glow plug control module providing inadequate voltage to the 'B' circuit

How to Fix It

Check your vehicle battery health first. A battery that drops below 12 volts at rest can trigger low-voltage codes throughout the vehicle. Have the battery tested for free at most auto parts stores.

Diesel engines draw huge amounts of current to spin the starter. A marginal battery fails under load and causes widespread electrical codes.
Locate the relay for glow plug circuit 'B'. On V-engine diesels, there may be separate relays for each bank. Test or replace the 'B' circuit relay — it is the cheapest part to swap and a frequent failure point.

Check your vehicle's fuse box diagram to identify the correct relay. Some vehicles label them clearly.
Inspect the wiring harness on the 'B' bank side of the engine. Pay particular attention to connectors at each glow plug. Clean any corroded terminals with electrical contact cleaner and reseat all connectors firmly.

Corrosion builds up inside connectors over time and adds resistance, causing voltage drop.
Test each glow plug on the 'B' bank with a multimeter. A plug reading near 0 ohms is shorted and will drag the circuit voltage down. Replace any shorted plug immediately.

A single shorted plug is enough to cause P0386 across the entire 'B' circuit.
If battery, relay, wiring, and plugs all pass, have the glow plug control module tested. A module that cannot regulate output voltage for the 'B' circuit will need replacement.

Confirm the module is faulty before purchasing — modules cost $80-$300.

When to Call a Professional

If replacing the relay does not clear the code, take the vehicle to a diesel specialist. Testing individual glow plugs for a short requires a multimeter and knowledge of the circuit. On high-mileage engines, a glow plug that snaps off during removal is a risk — professional shops have extraction tools. Expect $100-$200 for glow plug system diagnosis at a diesel shop.

Frequently Asked Questions

How do I know which side of the engine is circuit 'B'?

On most V-engine diesel trucks and SUVs, circuit 'A' is the driver-side bank and circuit 'B' is the passenger-side bank. However, this varies by manufacturer. Check your service manual or look up your specific engine on a diesel owner forum. Knowing the right bank tells you exactly which plugs and which wiring to inspect.

Can I drive with P0386?

Short distances in mild weather, yes. The 'A' circuit glow plugs may still heat the other bank, giving you a partial cold-start capability. In freezing temperatures, half your glow plugs not working means a significantly harder cold start — lots of cranking, white exhaust smoke, and rough running until warm. Fix it before cold weather arrives.

How much does fixing P0386 cost?

Battery: $100-$250. Glow plug relay: $15-$40. Wiring connector repair: $50-$200. Glow plugs per plug: $10-$30 each. Full set replacement with labor: $150-$400 for one bank. Glow plug control module: $80-$300.