OBD-II Code P0382 — Glow Plug / Heater Circuit 'A' Low

What Does This Error Mean?

P0382 means the glow plug circuit on your diesel engine is receiving less voltage than expected. The computer sees the voltage drop below the normal range on glow plug circuit 'A'. This usually points to a short to ground, a weak relay, or bad wiring. Your glow plugs may not be heating fully, which can lead to hard cold starts and white smoke from the exhaust. This code only applies to diesel engines.

Affected Models

Diesel vehicles 1996+
Ford Power Stroke diesel trucks
Chevy/GMC Duramax diesel trucks
Dodge/Ram Cummins diesel trucks
Mercedes and BMW diesel cars

Common Causes

A shorted glow plug pulling the circuit voltage down
A failed glow plug relay not delivering full voltage to the circuit
Corroded or damaged wiring creating excessive resistance
A weak vehicle battery not providing enough voltage during the glow cycle
A faulty glow plug control module outputting low voltage

How to Fix It

Check your battery first. Use a multimeter or have the battery tested at an auto parts store. A weak battery that drops below 12.2 volts at rest can cause low-voltage codes throughout the vehicle, including the glow plug circuit.

Diesel engines are very hard on batteries. If yours is over 4 years old, replace it regardless.
Locate and inspect the glow plug relay in the under-hood fuse box. Test it or swap it with an identical relay. A failing relay may deliver reduced voltage to the glow plug circuit even when it partially activates.

A new relay is a $15-$40 fix — always test this before replacing expensive plugs.
Inspect the wiring harness and connectors going to the glow plugs. Look for melted insulation, green corrosion, or chafing where the wire rubs against metal. A short to ground in this harness will pull the voltage low.

Pay close attention to any spots where the harness passes near the exhaust manifold — heat damage is very common here.
Test each glow plug for resistance with a multimeter. A normal plug reads 0.5–2 ohms. A reading near zero ohms (short circuit) means that plug is dragging the circuit down and needs to be replaced.

Even one shorted plug can cause the low-voltage code for the entire circuit.
If all plugs, wiring, and the relay test fine, the glow plug control module may be sending low voltage. Have a diesel specialist confirm this with a scan tool that can read glow plug module output before replacing it.

Control modules cost $80-$300 — confirm the diagnosis before spending that money.

When to Call a Professional

If you have tested and replaced the relay and the code returns, have a diesel mechanic test each glow plug individually. A shorted glow plug dragging the whole circuit down needs professional diagnosis. Bring your vehicle in cold if possible — some faults only appear when the engine is cold. Expect $100-$200 for a full glow plug system diagnosis.

Frequently Asked Questions

What is the difference between P0382 and P0383?

P0382 means the glow plug circuit voltage is too LOW. P0383 means it is too HIGH. Both indicate the voltage is outside the expected range. Low voltage (P0382) usually means a short to ground or a weak relay. High voltage (P0383) usually means a short to power or a wiring problem.

Will low glow plug voltage cause hard starting?

Yes — if the plugs are not getting enough voltage, they will not heat up fully. This means the combustion chamber is not warm enough to ignite fuel reliably. You may experience extended cranking, white smoke, and rough running when cold. The worse the weather, the more noticeable the problem will be.

How much does it cost to fix P0382?

A battery replacement runs $100-$250 depending on the size needed for your diesel. A glow plug relay is $15-$40. Wiring repairs vary widely — a simple connector fix might be $50, but replacing a harness section can be $200-$500. Replacing all glow plugs with labor runs $150-$600.