What Does It Mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to all vehicles manufactured after 1996 (Volkswagen, Audi, Isuzu, Dodge, Ford, etc.). Although generic, specific repair steps may vary by make/model.
When I encounter a stored P0381 code, I know the Powertrain Control Module (PCM) has detected a malfunction in the glow plug heater indicator circuit. This code is used exclusively in diesel engine vehicles. Diesel engine glow plugs are used to increase cylinder temperature in diesel engines, especially during cold start conditions.
High compression is used to ignite fuel in diesel engines. Since compression ratios are so extreme, diesel engine blocks are typically denser and heavier than gasoline engine blocks. This can result in lower cylinder temperatures that make diesel engines harder to start, particularly in cold weather. Since diesel engines achieve combustion without spark plugs, cylinder temperature is crucial for cold start efficiency.
Cylinder temperature increases when voltage is applied to glow plugs and they literally begin to glow. Most OBD-II equipped vehicles use one glow plug per cylinder, though some applications use fewer plugs strategically placed in the intake manifold or cylinder head.
Glow plug heater voltage is normally controlled by the PCM, but some vehicles also feature a standalone glow plug heater controller. Others have a glow plug heater controller integrated into the PCM. The glow plug controller uses input signals from the engine coolant temperature sensor and ambient air temperature sensor to calculate glow plug activation strategy. When the controller recognizes conditions warrant glow plug activation, an output signal is sent to a glow plug timer relay that allows battery voltage to reach the glow plugs. Since glow plugs are grounded through the engine block (or cylinder head), they become very hot and begin to glow when battery voltage is applied. Glow plug operation only occurs when engine temperature is below programmed temperature.
The glow plug heater control circuit includes the glow plug indicator. The glow plug indicator is a warning light integrated into the dashboard that illuminates when glow plugs/heaters are operating. During cold start conditions, the glow plug light typically comes on after the ignition switch is turned to ON position, and goes out before starting the engine. This brief period allows energized glow plugs to heat the cylinders before attempting to start the engine. The glow plug indicator is designed to alert the driver that glow plugs are functioning.
If the PCM detects a problem in the circuit controlling the glow plug/heater indicator, a P0381 will be stored and a malfunction indicator lamp may illuminate.
Severity and Symptoms
Glow plug operation will likely be disabled if a P0381 code is stored, so it should be considered a serious condition.
Symptoms of a P0381 code may include:
- Delayed engine starting, especially during extremely cold weather
- Excessive exhaust smoke
- No glow plug indicator
- Constant glow plug indicator illumination
Possible causes of this engine code include:
- Faulty glow plug indicator lamp bulb
- Defective glow plug/heater relay
- Malfunctioning glow plug/heater controller
- Open or shorted glow plug/heater indicator circuit
- Faulty instrument cluster circuit board
Diagnostic and Repair Procedures
A good starting point is always checking Technical Service Bulletins (TSBs) for your specific vehicle. Your problem may be a known issue with a known fix published by the manufacturer and can save you time and money when diagnosing.
You’ll need a scan tool, digital volt/ohmmeter (DVOM), and reliable vehicle information source (like All Data DIY) to diagnose a P0381 code.
I typically begin my diagnosis with a visual inspection of all wiring and connectors related to the system. In this case, I would also test all fuses and fuse links related to glow plugs/heaters, gauges, and indicators.
I would continue by connecting the scan tool to the vehicle’s diagnostic connector to retrieve stored codes and freeze frame data. I would write down this information. If this code proves intermittent, it may prove helpful. Then I would clear the codes and operate the glow plug/heater to see if it immediately resets. If P0381 resets, consult your vehicle information source for system relay design specifications. Use the DVOM to test system circuits and relays. Perform necessary repairs and retest the system.
If the glow plug heater indicator doesn’t illuminate, suspect an open circuit, faulty relay, or bad glow plug timer controller.
If the glow plug heater light stays on, suspect a short circuit (to power) or faulty instrument cluster circuit board.
If the glow plug indicator appears to function normally and P0381 continues to reset, suspect a faulty glow plug or open glow plug/heater circuit.
Additional Diagnostic Notes:
A simple method for testing glow plugs is to remove them from the engine, ground them, and apply 12 volts. If the glow plug begins to glow, it’s working properly. If not, replace the glow plug