How to test diesel glow plugs: resistance, current and control module
Glow plugs heat the combustion chamber for cold starts, and on many engines keep glowing after the start (post-glow) to reduce noise, smoke and emissions. Failed plugs cause hard cold starts, white smoke and rough running for the first minute, often with the glow light flashing or a code. Plugs that have been in for a long time can seize and snap when removed, so warm the engine and use the correct procedure.
Updated · written by ipacen; typical values, not manufacturer data
Disconnect the feed and measure each glow plug to ground: a working 12 V metal glow plug typically reads well under 2 Ω (often about 0.5–1.5 Ω), and an open circuit means it has failed. Compare all plugs: one far higher than the others is suspect. With a current clamp, each plug typically draws a high current at first, often about 10–25 A, that settles after a few seconds. Many modern engines use low-voltage plugs controlled by a module: check the specification before applying battery voltage to any plug.
Typical values
| Check | Typical good | Suspect |
|---|---|---|
| Resistance, 12 V metal plug (cold) | Typically about 0.5–1.5 Ω | Open (OL) or much higher than the others |
| Current per plug, first seconds | Often about 10–25 A, falling as it heats | Zero (open) or far from the others |
| Voltage at the plug during glow | Battery voltage (relay) or a pulsed feed from the module | No voltage: relay, module, fuse or wiring |
| Low-voltage plugs | Rated well under 12 V, fed by the module | Destroyed by direct battery voltage |
Typical ranges, not manufacturer data: check the specification for the engine.
Wiring: what you should measure
Glow plug circuit
| Wire | What you should see |
|---|---|
| Plug terminal | Feed from the relay or control module; the plug body grounds through the cylinder head |
| Control module | Battery supply, ground, a control signal from the engine ECU and one output per plug (on most modern systems) |
Before you start
- Read the fault codes: many glow plug modules report each plug separately.
- Check the battery: a weak battery makes a good glow system look bad.
- Find out whether the engine uses 12 V plugs or low-voltage plugs with a module.
Multimeter
- Disconnect the feed from each plug and measure resistance from the terminal to the engine; zero your meter leads first because the values are low.
- Ignition on (glow phase): measure voltage at the plug terminals.
Current clamp
- Clamp the main feed: total current should equal the number of plugs times the current of one, falling as they heat.
- Clamp each plug feed in turn and compare.
Removal
- Remove plugs with the engine warm, after soaking the threads, using the correct torque and turning back if it binds; a snapped plug means a head-off or a special extraction job.
What it looks like on a scope
Typical shapes drawn by ipacen, not a capture from one car: amplitude and timing vary with the engine, the sensor and the rpm.
What the readings tell you
| You find | It usually means |
|---|---|
| One plug open circuit | Failed glow plug: replace it and check the terminal connection |
| No voltage at any plug | Relay, control module, fuse or ECU control |
| Module code but the plugs measure fine | Module output or wiring to that plug |
| Rough running only for the first minute after a cold start | Glow plug or post-glow fault; also check injectors (leak-off) and compression |
Questions mechanics ask
How do you test glow plugs with a multimeter?
Disconnect the feed, zero the meter leads and measure from each plug terminal to the engine. A good 12 V plug typically reads well under 2 Ω; an open circuit (OL) is a failed plug. Compare all plugs: one much higher than the rest is suspect.
Can I test glow plugs by connecting them to 12 V?
Only if they are 12 V plugs, and only briefly with the plug out of the engine. Many modern engines use low-voltage plugs that a module drives with a pulsed feed; direct battery voltage burns them out.
Will a diesel start with one bad glow plug?
Usually yes, especially when warm, but cold starts get harder, with more smoke and rough running at first. With several failed plugs, a cold start may be impossible.
Values are typical, not manufacturer data. Always follow the service information for the vehicle and work safely.