Many diesels split their glow plugs into two groups, often one per bank. If one group stops heating, the engine starts hard and runs rough on cold mornings. A P0382 code means the engine computer found a problem in glow plug or heater circuit B.
This guide explains how the circuits are split, what usually fails on circuit B and how to test the plugs, relay or controller and wiring.
P0382 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 3 | Ignition system or misfire | The fault involves the ignition system, misfire detection or engine position sensors. |
| 82 | Specific fault | Glow Plug/Heater Circuit B Malfunction |
What the P0382 Code Means
The SAE J2012 standard defines the P0382 code as Glow Plug/Heater Circuit B Malfunction. Glow plugs are small electric heaters threaded into the cylinder head. When the engine is cold, the powertrain control module (PCM) powers them through a relay or glow plug control module so the diesel can start.
On engines with two glow plug circuits, circuit B usually feeds the second group of plugs, often the second bank on a V engine. Some systems use circuit B for a second relay, and some use it for an intake air heater. Check your service manual to see what circuit B covers on your engine.
The PCM or controller watches current and voltage on each circuit. When circuit B shows too little current, no voltage or a short, the code sets.
Symptoms of a Glow Plug Circuit B Fault
Symptoms are worst in cold weather. Common signs of a P0382 code include:
- Hard starting or long cranking when cold
- White smoke after a cold start
- Rough idle or misfire on one bank until warm
- Check engine light on
- Engine knock right after a cold start
If only one group of plugs fails, the engine may still start, but the cylinders on the cold side misfire and smoke until they warm up.
Common Causes of the P0382 Code
These are the usual causes:
- Burned-out glow plugs on circuit B. One or more plugs in the group have failed.
- Failed relay or controller output. The second output of a controller can fail on its own.
- Damaged wiring for circuit B. Connectors can burn or corrode on the head.
- Blown fuse or fusible link. For the second circuit.
- Corroded battery cables or grounds. Voltage drops under high current.
- PCM fault. Rare.
Tools You Will Need
You can do most of this testing at home with basic tools:
- OBD-II scan tool that can command the glow plugs
- Digital multimeter
- Clamp-on amp meter
- Glow plug socket and torque wrench
- Wiring diagram for your engine
How to Track Down the P0382 Code
Disconnect the batteries before you unplug or remove any glow plug wiring, since these circuits carry high current.
Identify Circuit B
Use the wiring diagram to see which glow plugs, relay output or heater circuit B feeds on your engine. Many scan tools also show status for each plug or circuit.
Measure Circuit B Current
With a clamp-on amp meter around the circuit B feed, command the glow plugs on with a cold engine. Compare the current with the number of plugs on that circuit times the current per plug. Low current points to failed plugs.
Test Each Plug on Circuit B
Unplug each glow plug on circuit B and measure resistance to the engine block. Most read about 0.5 to 2 ohms cold. An open reading means a burned-out plug.
Check the wiring and connectors to each plug for burns, corrosion and broken wires.
Check the Relay or Controller Output
Check that battery voltage reaches the circuit B feed when the system is commanded on. No output with a good command points to the relay or controller.
Repairs That Clear the P0382 Code
Match the repair to what your tests found. Then clear the code and drive under the same conditions that set it.
- Replace burned-out glow plugs on circuit B, or the full set on high-mileage engines.
- Replace the relay or glow plug controller if circuit B has no output.
- Repair damaged wiring or connectors.
- Replace a blown fuse or fusible link after checking for a short.
- Clean battery cables and grounds.
Glow plugs can seize in the cylinder head. Warm the engine, use penetrating oil and the right socket, and torque new plugs to specification. Never force a stuck plug.
Common Mistakes to Avoid
These mistakes waste money or leave the real fault in place:
- Testing circuit A plugs when circuit B set the code
- Forcing a seized glow plug
- Over-tightening new glow plugs
- Replacing plugs when the controller output has failed
Repair Cost Breakdown
The cost to fix a P0382 code depends on the cause. This table shows typical ranges for parts and labor.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Glow plug, each | $15 to $60 | $80 to $200 |
| Glow plug set | $60 to $300 | $300 to $900 |
| Glow plug relay or controller | $50 to $300 | $200 to $600 |
| Glow plug wiring or connector | $10 to $80 | $100 to $350 |
Labor depends on where the plugs sit. On some V8 diesels, plugs on one bank are much harder to reach than the other, which raises the cost.
Can You Drive with a P0382 Code?
Usually, yes, in mild weather. A warm diesel often starts without glow plugs. In cold weather, though, it may crank for a long time, start rough or not start at all.
Long cranking wears the starter and drains the batteries. Fix the fault before winter, and avoid driving in very cold conditions until it is repaired.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0380 | Glow Plug/Heater Circuit A Malfunction |
| P0381 | Glow Plug/Heater Indicator Circuit Malfunction |
| P0670 | Glow Plug Module Control Circuit |
| P0671 | Cylinder 1 Glow Plug Circuit |
Getting Glow Plug Circuit B Working Again
A P0382 code means the PCM found a fault in glow plug or heater circuit B. Burned-out plugs, failed controller outputs and damaged wiring cause most cases.
- Find out which plugs circuit B feeds.
- Measure current on circuit B.
- Test each plug on that circuit.
- Check the relay or controller output.
Frequently Asked Questions
Is Circuit B the Same as Bank 2?
On many V engine diesels, yes, circuit B feeds the glow plugs on the second bank. On other engines, it may be a second relay output or an intake air heater. The wiring diagram for your engine shows what it covers.
How Many Glow Plugs Does a Diesel Have?
Most diesels have one glow plug per cylinder, so a V8 diesel has eight. Some engines also have an intake air heater instead of, or along with, glow plugs. Your service manual shows what your engine uses.
Can I Test Glow Plugs with a Multimeter?
Yes. Unplug the glow plug and measure from its terminal to the engine block. Most read about 0.5 to 2 ohms when cold, but check your service manual. An open reading means the plug has burned out.
Should I Replace All the Glow Plugs at Once?
Many owners do, since glow plugs wear at similar rates and some are hard to reach. If one has failed on a high-mileage engine, the rest may follow soon. Replacing them as a set can save labor later.
Do Glow Plugs Wear Out?
Yes. Each heat cycle wears the heating element a little, and most glow plugs eventually burn out. Many last 60,000 to 100,000 miles or more, depending on the engine and climate. Some newer engines use ceramic glow plugs that last longer.
Why Does Only One Side of the Engine Misfire When Cold?
If circuit B feeds one bank and its plugs are not heating, those cylinders start cold and misfire until combustion heat builds up. The other bank fires normally. This uneven start often shows as white smoke from one side and a rough idle for the first minute.
Can I Drive with the P0382 Code?
Usually, yes, especially in warm weather. Expect harder cold starts and some smoke until the engine warms up. Fix it before cold weather, since long cranking strains the starter and batteries.