OBD-II Code P0384 — Glow Plug / Heater Circuit 'A' Intermittent

What Does This Error Mean?

P0384 means the glow plug circuit on your diesel engine is working sometimes but not reliably. The computer is detecting the fault only occasionally — it comes and goes. Intermittent faults are usually caused by a loose connection, a cracked wire, or a relay on its way to failing. You might only notice a hard start occasionally, especially on cold mornings. Because the fault is inconsistent, it can be harder to diagnose than a straight failure.

Affected Models

Diesel vehicles 1996+
Ford Power Stroke diesel trucks
Chevy/GMC Duramax diesel trucks
Dodge/Ram Cummins diesel trucks
VW, Audi, and BMW diesel vehicles

Common Causes

A loose or corroded connector on a glow plug or at the relay
A cracked wire in the harness that makes contact sometimes but not always
A glow plug relay that is starting to fail and drops out randomly
A glow plug with intermittent internal failure that only acts up when cold
Thermal expansion causing a poor connection to open when the engine is hot

How to Fix It

Start by checking all glow plug connectors and the relay connector for looseness or corrosion. Wiggle each connector while monitoring with a scan tool if possible. A connector that shifts the reading is your culprit.

Intermittent faults are almost always a connection problem — start here before replacing parts.
Inspect the wiring harness for any signs of cracking, brittleness, or chafing. Diesel engine bays run very hot and plastic wiring insulation can crack and cause faults that only show up under certain conditions.

Focus on areas near the exhaust and the turbocharger where heat damage is most common.
Replace the glow plug relay even if it appears to be working. A relay that is starting to fail may test fine when cold but drop out when it gets warm. Since relays are cheap ($15-$40), replacing it is a logical first step.

Relays have a limited lifespan and high-mileage diesel trucks often have original relays.
Test each glow plug resistance when the engine is cold. An intermittently failing plug may show normal resistance when tested but still fail to heat properly in real conditions. Replace any plug that reads outside the 0.5–2 ohm range.

If the engine has over 100,000 miles, replacing the full set is good preventive maintenance.
Clear the code and monitor for recurrence. If the code returns only on cold mornings, the fault is temperature-related. Log when it happens to give your mechanic better information for diagnosing the root cause.

Some advanced scan tools can log glow plug circuit voltage over time so you can catch the drop when it happens.

When to Call a Professional

Intermittent faults are notoriously difficult to track down without the right scan tools. If you cannot find a loose connection or failing relay, take it to a diesel mechanic. Asking them to test the system when the engine is cold gives you the best chance of replicating the fault. Expect $100-$180 for diagnosis of an intermittent electrical fault.

Frequently Asked Questions

Why is an intermittent fault harder to fix than a permanent one?

Because the problem has to be present to find it. If a wire is only broken under certain conditions — like when it is cold, or when the engine vibrates — it may appear normal when you are testing it. The best strategy is to replicate the conditions when the fault occurs. If it only happens on cold mornings, test it first thing in the morning before the engine warms up.

Will P0384 cause my diesel to fail an emissions test?

In most states and countries, a stored OBD-II trouble code will cause an automatic fail on an emissions test. Even if the vehicle runs fine, the code must be cleared and the monitors must complete before a test. Fix the underlying problem first, then clear the code and drive normally for a few days before testing.

How much does it cost to fix P0384?

Cleaning and reseating connectors is a free DIY fix. A new glow plug relay is $15-$40. Replacing all glow plugs with labor is $150-$600. Wiring repair costs depend on the damage — anywhere from $50 to $400. Diagnosis at a diesel shop runs $100-$180.