The glow plug light tells you the diesel preheat system has a fault. It usually points to a bad glow plug, relay, or wiring issue. Battery voltage drops can also trigger the indicator.
Checking resistance and scanning for codes confirms the cause.
Per SAE J1235, glow plug resistance should read between 0.5 and 2 Ω at 20 °C as of 2026. If the reading is outside that range, replace the plug. A quick OBD‑II scan often reveals DTC P0670‑P0679.
This data point leads us straight into the diagnosis steps.
Why Is the Glow Plug Light On
The amber light signals a fault in the diesel preheat system. It usually means a glow plug, relay, or wiring issue. Battery voltage drops can also trigger the indicator.
Check resistance and scan for codes to confirm.
Decision Tree: 3‑Step Diagnosis Path
Step 1 – Verify battery voltage and relay operation
- Connect a multimeter to the battery terminals. Look for 12.6 V with the engine off. Low voltage can mimic a glow plug fault.
- Tap the relay gently while the light is on. If the light flickers, the relay may be stuck.
Step 2 – Test glow plug resistance (0.5 – 2 Ω)
- Remove the glow plug using a socket and heat‑resistant gloves.
- Measure resistance at the plug terminals. Values outside 0.5‑2 Ω mean replace the plug.
Step 3 – Scan ECU for DTCs and decide on component replacement
- Plug an OBD‑II scanner into the port and read all codes.
- Follow the manufacturer’s service manual for the specific DTC.
- Replace the faulty part with an OEM‑matched component.
| Step | Tool Needed | Spec to Check |
|---|---|---|
| Battery test | Multimeter | ≥ 11.5 V |
| Relay test | Multimeter | Continuity |
| Plug test | Multimeter | 0.5‑2 Ω |
| Scan | OBD‑II scanner | DTC range P0670‑P0679 |
External reference: SAE J1235 outlines these test procedures.
Internal link for scanning: scanning for codes.
Common Mistakes & Safety Pitfalls
- Skipping the battery voltage check leads to unnecessary part swaps.
- Using a multimeter set to the wrong range gives false resistance readings.
- Ignoring corroded harness connections can cause intermittent lights.
- Working on hot plugs without heat‑resistant gloves burns hands.
- Forgetting to disconnect the battery before testing can damage the ECU.
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Frequently Asked Questions
Why does the light flash after a cold start?
The ECU flashes the light while the glow plugs heat. If the preheat cycle finishes and the light stays on, a fault exists.
Can a weak battery trigger the glow plug light?
Yes. Low battery voltage can make the ECU think the glow plugs failed. Test voltage before replacing parts.
When should I replace the relay instead of the plug?
Replace the relay if continuity is lost or the coil draws abnormal current. A stuck relay often causes repeated light activation.
Final Decision Guide: Fix, DIY, or Call a Pro
- If all tests pass, reset the system and clear DTCs.
- If a plug is out of spec, replace it with an OEM part.
- If the relay sticks, replace or repair per the service manual.
- If the ECU shows a persistent DTC, seek a dealer or ASE‑certified shop.
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