What are the symptoms of a bad glow plug relay?
Common signs of a bad glow plug relay include hard cold starts, long cranks, rough idle, and a check engine light with glow plug fault codes.
The glow plug relay powers the glow plugs in diesel engines to heat the combustion chamber during cold starts. When it fails, the plugs may not heat sufficiently or may stay energized too long, leading to starting and running problems. This article explains the symptoms, how to diagnose the issue, and steps you can take to address it.
What the glow plug relay does
The glow plug relay acts as a switch controlled by the engine management system. When you turn the key or start the engine, the relay energizes the glow plugs to heat the combustion chambers during cold starts. If the relay sticks, fails to energize, or energizes continuously, the plugs’ performance suffers and starting reliability declines.
Symptoms of a bad glow plug relay
Symptoms listed below are typical indicators, but they may be caused by faulty glow plugs, wiring, or sensors too. A professional diagnosis is recommended to pinpoint the root cause.
- Difficulty starting in cold weather or after the vehicle has sat for several hours.
- Long cranks or multiple attempts required before the engine fires.
- Rough idle or uneven running shortly after starting, especially until the engine warms up.
- White or blue-tinged exhaust smoke on startup that clears after the glow plugs heat.
- Engine misfires or hesitation during acceleration due to incomplete combustion on cold starts.
- Check Engine Light with glow plug-related codes such as P0380 (glow plug/heater circuit malfunction) or P0670–P0676 (glow plug circuit faults for specific cylinders).
- No audible relay click or energizing sound when the glow cycle is supposed to begin, or conversely a continuous glow after starting that drains the battery.
- Unexplained battery drain or frequent dead battery in cold weather due to a relay remaining closed.
When you notice these symptoms, it's important to diagnose promptly. The underlying issue could be the relay, the glow plugs, wiring, or the engine control module's signal to the relay.
How to diagnose a bad glow plug relay
Use a structured approach to verify the relay and the glow plug circuit. The following steps help isolate the relay as the culprit versus failing glow plugs or harness problems.
- Check fuses and wiring: verify the fuse for the glow plug system is intact and that wiring to the relay and glow plugs shows no corrosion or damage.
- Listen for relay operation: with the ignition in pre-heat or crank position (depending on model), listen for a distinct click from the relay. No click can indicate a failed relay or lack of control signal.
- Test the relay coil: measure the resistance across the relay coil terminals with a multimeter and compare to the manufacturer’s specification.
- Measure voltage at the relay input/output: ensure the relay is receiving 12V (or the system voltage) when energized and that the output to the glow plugs is present during the pre-heat phase.
- Test glow plugs individually: remove the plug connectors and measure each glow plug’s resistance against the spec. Very high resistance or open circuit means a failed glow plug.
- Check the control signal: verify that the engine control module (ECM/PCM) is sending the correct signal to energize the relay during cold starts. This may require a professional scan tool.
- Perform a current draw test: abnormal current draw through the glow plug circuit can indicate a stuck relay or shorted glow plug.
- Consider coding/test sequences: some vehicles require a specific crank/temperature profile or battery voltage threshold for the glow cycle; verify procedures in the service manual.
Based on these checks, you can determine whether the relay, the glow plugs, or the associated wiring needs replacement.
What to do if you suspect a bad glow plug relay
If diagnostics point to a faulty relay, replacement is typically straightforward, but you should also inspect or replace glow plugs if they are worn or damaged. Use OEM or quality aftermarket parts and follow the vehicle’s service manual for torque specifications and wiring procedures. If you’re not comfortable performing electrical diagnostics, contact a qualified mechanic or diesel specialist.
Summary
A bad glow plug relay often shows up as cold-start trouble, extended cranking, rough idling, and diagnostic codes related to the glow plug circuit. Diagnosing involves checking fuses and wiring, confirming the relay energizes, testing the coil and supply voltage, and verifying glow plugs individually. Replacing the relay and any faulty glow plugs, while ensuring clean wiring, usually resolves the issue and restores reliable cold starts.
