Hard starts in cold weather are one common diesel complaint.
And a lot of times, it comes back to one thing:
Glow plugs not doing their job.
Glow plugs are responsible for heating the combustion chamber so the engine can start when it’s cold. If one or more fail, you’ll start seeing issues like long crank times, white smoke, or rough startup.
The good news is : testing them is straightforward if you follow the right process.
Common Signs You May Have Bad Glow Plugs
Before you even grab a meter, there are a few patterns that point toward glow plug issues:
- Hard Cold Starts
- Excessive White Smoke At Startup
- Rough Idle When Cold
- Engine Starts Fine When Warm But Not Cold
That last one is key. If the engine runs fine once it’s warm, glow plugs are often part of the problem.
The Most Reliable Way to Test Glow Plugs (Using a Multimeter)
The most accurate method is a resistance test. This tells you if the glow plug is still functioning internally.
Step-by-Step Process
- Make Sure the Engine Is Cold
Always test glow plugs on a cold engine to avoid false readings. - Disconnect the Glow Plug Wire
This is critical. If you don’t disconnect it, you’ll be reading the entire circuit instead of a single plug. - Set Your Multimeter to Ohms (Ω)
You’re measuring resistance. - Connect Your Leads
- Negative lead → Engine ground
- Positive lead → Top terminal of the glow plug
- Read the Resistance Value
What Readings Mean
- 0.5 to 2 ohms (sometimes up to ~6 ohms depending on engine) → Good glow plug
- Very high resistance or OL (open loop) → Failed glow plug
- No reading at all → Completely dead plug
The key here is consistency.
If you test all cylinders and one reads differently, that’s usually your problem.
Testing Glow Plug Power Supply (Often Overlooked)
A lot of people stop at the plug – but that’s only half the system.
Glow plugs need proper voltage to work.
Quick Voltage Check
- Set multimeter to DC voltage
- Ground the negative lead
- Touch the positive lead to the glow plug terminal
- Turn the key to the “on” position
You should see close to battery voltage (~12V) during the glow cycle
If you don’t:
- The issue could be the relay
- The control module
- Or wiring in the circuit
This is where misdiagnosis happens a lot: replacing glow plugs when the system isn’t powering them.
Alternative Methods (Quick Field Checks)
There are a couple quicker methods shops sometimes use, but they’re less precise.
Current Draw Test
Each glow plug should draw roughly 10–20 amps when energized. Low or no draw usually indicates failure.
Bench Test (Use With Caution)
Applying power briefly to see if the tip heats up can work – but some manufacturers advise against it due to risk of overheating or damage.
Where Testing Goes Wrong
This is where a lot of repeat issues come from.
Testing Without Disconnecting the Circuit
You end up reading all plugs together – not helpful.
Ignoring Voltage Supply
You replace good glow plugs, and the problem comes back.
Replacing One Plug Instead of the Set
If one failed, the others are usually not far behind.
Not Looking at the Root Cause
Glow plugs fail for a reason:
- Poor combustion
- Fuel system issues
- Carbon buildup
If those aren’t addressed, you’ll see repeat failures.
Why Glow Plug Testing Matters for Diesel Engines
Glow plugs aren’t just a cold-start convenience.
When they’re not working properly, it can lead to:
- Incomplete Combustion
- Excess Fuel Washdown
- Increased Engine Wear Over Time
For fleets and working trucks, that turns into:
- More downtime
- Higher maintenance costs
- Shorter engine life
Final Takeaway
Testing glow plugs isn’t complicated – but doing it correctly makes all the difference.
The most reliable approach is:
- Check resistance on each plug
- Verify voltage supply
- Compare results across cylinders
Because in most cases, the issue isn’t just one failed part.
It’s a system problem.
Need Help Diagnosing a Hard-Start Issue?
If your engine is:
- Hard to start in cold weather
- Smoking at startup
- Running rough when cold
It’s worth stepping back and looking at the full system: not just the glow plugs.
Call 844-304-7688
We’ll help you diagnose the issue correctly the first time.
From diagnosis through delivery – we’re Highway and Heavy Parts.
