Jeep WK Grand Cherokee OM642 Glow plug Fault
- Gordon Atkinson
- May 30
- 3 min read
OM642 Glow Plug Fault: Causes, Symptoms, Diagnosis, and Fixes
The Mercedes-Benz OM642 diesel engine is known for its smooth performance, strong torque, and long service life. However, one recurring issue that many owners encounter—especially in colder climates—is the glow plug fault. While it may sound minor at first, it can lead to hard starting, warning lights, and in some cases, expensive repairs if ignored.
This article breaks down what the OM642 glow plug fault is, why it happens, how to diagnose it properly, and what your repair options are.
What is the OM642 Glow Plug System?
The OM642 is a 3.0L V6 turbo-diesel engine used in many Mercedes models, including the ML, GL, E-Class, Sprinter, and others. It is also used in the Jeep WK Grand Cherokee.
Unlike petrol engines, diesel engines rely on glow plugs to preheat the combustion chamber. This helps the engine start efficiently, especially in cold temperatures.
Each cylinder typically has its own glow plug, controlled by a glow plug control module (relay unit).
Common OM642 Glow Plug Fault Symptoms
When something goes wrong in the glow plug system, you’ll usually notice one or more of the following:
1. Glow Plug Warning Light
A dashboard warning light shaped like a coil or “check engine” light appears.
2. Hard Starting in Cold Weather
The engine cranks longer than usual or struggles to start on cold mornings.
3. Fault Codes Stored
Common diagnostic trouble codes include:
P0670–P0676 (glow plug circuit faults)
P0671–P0676 (specific cylinder glow plug failure)
Control module communication errors
4. Limp or Reduced Efficiency (less common)
While glow plug faults don’t usually cause limp mode directly, related electrical issues sometimes do.
Why OM642 Glow Plug Faults Happen
The OM642 engine is particularly known for glow plug issues due to a combination of heat, carbon buildup, and electrical stress.
1. Seized Glow Plugs
Carbon deposits around the glow plug can cause it to seize in the cylinder head. This is one of the most serious issues because removal can risk breaking the plug.
2. Faulty Glow Plug Control Module
The control module (relay unit) is a common failure point. When it fails, it may incorrectly report multiple glow plug faults at once.
3. Wiring Harness Damage
Heat and vibration in the engine bay can degrade wiring insulation or connectors.
4. Aging Glow Plugs
Over time, glow plugs naturally wear out and lose efficiency, especially after 80,000–150,000 km.
5. Water or Oil Contamination
Leaks from valve covers or poor sealing can damage electrical components.
How to Diagnose the Problem Properly
Before replacing parts, proper diagnosis is essential.
Step 1: Scan for Fault Codes
Use a Mercedes or Jeep-compatible diagnostic tool (such as Xentry or a high-quality OBD scanner).
Step 2: Identify Pattern of Failure
One cylinder fault → likely a single glow plug
Multiple cylinder faults → often control module or wiring
All cylinders reporting issues → very likely control unit failure
Step 3: Test Resistance
A multimeter can be used to check glow plug resistance. A failed plug usually shows an open circuit.
Step 4: Check the Control Module
Inspect for corrosion, heat damage, or communication faults.
Common Repairs and Solutions
1. replace the Glow plug Module.
On the OM642 the most common cause for any of the glow plug issues is the module. its easily accessible on the front of the engine and in 95% percent of cases is the culprit.
2. Replacing the Glow Plug
If accessible and not seized, replacing glow plugs is straightforward. However, caution is required due to the risk of breakage inside the cylinder head.
3. Extracting Seized Glow Plugs
If a glow plug breaks, specialized extraction tools or professional machining may be required. This can significantly increase repair cost.
4. Wiring Repairs
Damaged connectors or harness sections may need repair or replacement.
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