Drivers who encounter a diesel engine that refuses to fire on a cold morning often see the check‑engine light flash and experience extended cranking before the engine finally turns over. The vehicle may cough, run roughly, and then settle once the combustion chambers reach operating temperature. These signs point to a P0382 – Glow Plug/Heater Circuit B – fault. The power‑train control module (PCM) has detected an abnormal voltage condition in the secondary glow‑plug heater circuit, indicating either an open circuit, a short to ground, or a voltage outside the calibrated range. Early identification prevents repeated hard starts and protects the PCM from being forced to operate under abnormal electrical loads.
Corrosion, chafed harnesses, or loose connectors can raise the resistance in the heater‑circuit path. The PCM expects a specific voltage drop when the heater is energized; a high‑resistance fault mimics an open circuit, prompting P0382.
A damaged wire that contacts the engine block or chassis ground creates a short, pulling the heater voltage to near zero. Conversely, a short to power can push voltage above the PCM’s safe limit, both conditions triggering the fault.
Individual glow‑plug heaters contain a resistive element that heats the combustion chamber. Internal coil breakage or burnout eliminates the required resistance, resulting in an open‑circuit reading.
The PCM supplies the 12 V (or 24 V on some heavy‑duty diesels) to the heater circuit through an internal driver transistor. If the driver burns out or the PCM’s internal voltage regulator drifts, the PCM will log P0382 even though the external wiring and heater are intact.
– Connect a professional OBD‑II scanner capable of reading diesel-specific codes. Record the exact voltage the PCM reports for Heater B during the fault.
– Check for cracked insulation, corrosion, and loose connector pins. Verify that the connector latch is fully engaged.
– With the ignition off, disconnect the heater‑circuit connector. Measure resistance between the two pins; a typical glow‑plug heater reads 2–5 Ω (varies by manufacturer). An infinite reading indicates an open circuit; a near‑zero reading suggests a short.
– Re‑connect the connector, turn the ignition to the “on” position (engine not cranking), and measure voltage at the connector pins. Expect battery voltage (≈12.6 V). Any deviation beyond ±0.5 V may point to a PCM driver issue.
– If wiring checks out, remove the glow‑plug heater from the cylinder head (if serviceable) and test resistance directly. Replace any heater that reads open or exceeds the manufacturer’s upper limit.
– Use the scanner’s bi‑directional control to command the PCM to energize Heater B while monitoring voltage. If the PCM fails to supply voltage despite a good circuit, the internal driver is likely defective.
– If the fault originated from a recent software update or improper calibration, reflash the PCM with the latest manufacturer‑approved firmware. Verify that the updated map includes correct heater‑circuit parameters.
– Replace damaged wiring harness sections, connectors, or the glow‑plug heater as needed. If the PCM driver test confirms internal failure, replace the PCM. Ensure the replacement unit is VIN‑matched and programmed to the vehicle’s specifications.
Cost estimates
Service Recommendation: Most issues related to this fault are diagnosed and corrected through inspection, wiring repair, and calibration rather than module replacement. For modules not typically replaced through aftermarket suppliers, diagnosis and repair should be performed by a certified automotive technician with access to factory service information and tooling.