OBD-II Diagnostic Trouble Code
P0383 — Glow Plug/Heater Circuit A High
P0383 means the ECM sees glow plug or heater circuit A voltage or feedback higher than expected. This usually points to an open circuit, a relay stuck on, or a short to voltage on the control or feedback wire.
Severity 2/5
Truck-relevant
Service soon
Severity
Service soon
Glow Plug/Heater Circuit A High
Code
P0383
Category
Powertrain
Subsystem
Cold Start
Truck-relevant
Yes
What this means for truckers
P0383 (Glow Plug Circuit A High) — glow plug circuit shorted to power.
Before you replace parts — try this first
P0383 on the Cold Start system is most often glow plug relay or control module stuck on — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Hard cold starts if the glow plugs are not powered, or glow plugs left on too long if the relay is stuck, which can drain batteries and burn out plugs. Check-engine light on, and the wait-to-start lamp may act strangely.
Common causes
- CommonGlow plug relay or control module stuck on ($200–$700)
- CommonOpen glow plug circuit from failed plugs or broken wiring ($150–$900)
- OccasionalControl or feedback wire shorted to battery voltage ($100–$450)
- RareECM driver fault ($1200–$3500)
How to fix
Check whether the glow plugs stay powered after preheat ends
When to use: First step
Replace the stuck relay or glow plug control module
When to use: When power stays on after the command stops
Check the control and feedback wires for a short to voltage
When to use: When the relay tests good
Test each glow plug and replace failed ones
When to use: When the circuit reads open
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0300 — Random/Multiple Cylinder Misfire Detected
- → P0301 — Cylinder 1 Misfire Detected
- → P0302 — Cylinder 2 Misfire Detected
- → P0303 — Cylinder 3 Misfire Detected
- → P0304 — Cylinder 4 Misfire Detected
Frequently Asked Questions
Can I drive with code P0383?
P0383 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix P0383 on a commercial truck?
Total repair cost ranges $100–$3500 depending on the root cause. The cheapest fix is usually the first one listed below — Glow plug relay or control module stuck on — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0383 fail a DOT inspection?
P0383 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0383 and related codes?
P0383 is closely related to: P0300 (Random/Multiple Cylinder Misfire Detected), P0301 (Cylinder 1 Misfire Detected), P0302 (Cylinder 2 Misfire Detected), P0303 (Cylinder 3 Misfire Detected). These codes share the same subsystem (cold-start) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0383 is set without the companions, the issue is more isolated.
How do I prevent P0383 from coming back after a repair?
For P0383 (Glow Plug/Heater Circuit A High), prevention centers on the cold start system. Test glow plug system annually before winter. Use a battery in good condition — weak batteries make glow plugs work harder. Park indoors below -10°F if possible.