OBD-II Code P0385 — Glow Plug / Heater Circuit 'B' Malfunction

What Does This Error Mean?

P0385 is the same type of fault as P0380, but it affects the second glow plug circuit, labeled 'B'. Some diesel engines — particularly V6 and V8 diesels — split the glow plugs into two circuits, one per bank of cylinders. Circuit 'B' typically covers one bank of the engine. This means some of your glow plugs may not be heating when you start the engine cold. Expect hard cold starts and possible white smoke until this is repaired.

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

Failed glow plugs on the 'B' bank of the engine
A faulty glow plug relay dedicated to the 'B' circuit
Broken or corroded wiring in the 'B' circuit harness
A failed glow plug control module not activating the 'B' circuit
Corroded glow plug connectors on the 'B' bank cylinders

How to Fix It

Determine which bank of cylinders is circuit 'B' on your engine. For most V6 and V8 diesels, 'A' is the driver-side bank and 'B' is the passenger-side bank. Consult your service manual or a diesel forum specific to your vehicle.

Knowing which cylinders are affected tells you exactly where to focus your inspection.
Locate the glow plug relay for the 'B' circuit. Some engines have separate relays for each bank. Test or swap the relay to rule it out as the cause. A failing relay is the cheapest and easiest fix at $15-$40.

On some vehicles, one relay controls all glow plugs. On others, there is one per bank.
Inspect the wiring harness on the 'B' bank side of the engine. Look for heat damage near the exhaust, corrosion at connectors, or broken wires. Repair any damage found before testing the glow plugs themselves.

The passenger-side bank on many trucks runs very close to the hot side of the turbo system.
Test each glow plug on the 'B' bank with a multimeter. Normal resistance is 0.5–2 ohms per plug. Replace any plug reading outside that range. Consider replacing all plugs on that bank as a set.

If both banks are high mileage, replace all plugs at once to save on labor later.
If relay, wiring, and plugs all check out, the glow plug control module may not be activating the 'B' circuit. A diesel mechanic with a scan tool can command the glow plugs on and verify whether both circuits are being activated.

Some modules have separate outputs for each circuit and can fail on just one side.

When to Call a Professional

If you are not sure which bank of cylinders is the 'B' circuit on your specific engine, look it up in the service manual or ask a diesel mechanic. On inline engines, the distinction between 'A' and 'B' is less common. On V-type engines, 'A' is usually the driver's side bank and 'B' is the passenger side. A diesel specialist can test the entire glow plug system in one session for $100-$200.

Frequently Asked Questions

What is the difference between circuit 'A' and circuit 'B'?

On a V-configuration diesel engine, the glow plugs are often split into two separate circuits. Circuit 'A' controls one bank of cylinders, circuit 'B' controls the other. This design allows the computer to diagnose which side of the engine has the problem. If you get both P0380 and P0385 at the same time, the problem is likely the control module or the relay common to both circuits.

Can I drive my diesel truck with P0385?

Yes, in warm weather you can drive with limited impact. In cold weather, half your glow plugs not heating means the affected bank of cylinders will struggle to fire on cold starts. You may hear rough running, see white smoke from the exhaust, and experience power loss until the engine warms up. Repair it before winter.

How much does fixing P0385 cost?

A glow plug relay costs $15-$40. Individual glow plugs cost $10-$30 each. Replacing all plugs on one bank with labor runs $100-$350 depending on the engine. Wiring repairs are $50-$400 depending on severity. A glow plug control module is $80-$300.