P0388
Universal (All Makes) Vehicle (OBD-II)
Severity: MinorWhat Does This Error Mean?
P0388 means the 'B' glow plug circuit on your diesel engine is working inconsistently. The fault appears and disappears rather than staying on permanently. This makes diagnosis tricky — the circuit may test normal when you check it. Intermittent glow plug faults almost always come down to a loose connector, a cracked wire, or a relay starting to fail. You may only notice the problem as an occasional hard cold start.
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 loose connector on one of the 'B' bank glow plugs that loses contact intermittently
- A cracked wire in the 'B' circuit harness that opens under vibration or temperature changes
- A glow plug relay for the 'B' circuit that is beginning to fail
- A glow plug with internal deterioration that only acts up when cold
- Thermal expansion causing a marginal connection to open when the engine reaches operating temperature
How to Fix It
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Start by firmly pushing in and wiggling every connector on the 'B' bank glow plugs and the glow plug relay connector. A connector that moves the fault when wiggled is your problem. Clean it with electrical contact cleaner and reseat it.
Connectors are the number-one cause of intermittent faults — they expand and contract with temperature changes and can lose contact.
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Inspect the 'B' bank wiring harness for cracked or brittle insulation. Flex each section of wire gently by hand. Look for any spot where the wire bends sharply around a bracket or clips — these are prime locations for cracked insulation.
Diesel engine harnesses near the exhaust age quickly — the heat makes insulation brittle over time.
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Replace the glow plug relay for circuit 'B' even if it appears to be working. Intermittently failing relays test fine when cold but drop out unpredictably. At $15-$40, a new relay is cheap insurance.
A relay on a high-mileage diesel truck has often been in service for 100,000+ miles — they have a finite lifespan.
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Test the glow plugs on the 'B' bank when the engine is cold. An intermittently failing plug may show unusual resistance on a cold morning even if it tests fine at operating temperature.
Cold testing catches faults that a warm engine would mask.
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Clear the code and keep a log of when it returns. Note the outside temperature, whether the engine was cold or warm, and how long since the last start. This data helps a mechanic isolate what conditions trigger the fault.
Some advanced scan tools can capture glow plug circuit voltage over a drive cycle so you can see exactly when the fault occurs.
When to Call a Professional
Intermittent faults are the hardest type to diagnose because the problem must be present to find it. If you cannot locate a loose connector or failing relay, take the vehicle to a diesel specialist and describe exactly when the fault occurs. Testing the system from a cold start gives the best chance of catching the fault in action. Expect $100-$180 for intermittent fault diagnosis.
Frequently Asked Questions
Why does P0388 only happen on cold mornings?
Glow plugs only run during cold starts. When the engine is cold, the control module turns the plugs on to heat the combustion chamber before cranking. If a connection or plug is only marginal, it may fail right at this moment and set the code. Once the engine warms up, the glow plugs are no longer needed — so the fault disappears until the next cold start.
Is P0388 dangerous?
Not immediately. An intermittent glow plug fault usually means occasional hard cold starts rather than a total failure to start. But intermittent faults are progressive — they tend to get worse over time. Fix it now while it is still occasional rather than waiting until it becomes a no-start situation in the middle of winter.
How much does it cost to fix P0388?
Cleaning connectors is free. A glow plug relay is $15-$40. Wiring repair varies: $50-$400 depending on severity. Replacing glow plugs on one bank with labor: $150-$350. Diagnosis at a diesel shop: $100-$180.