Case Study
Mercedes S-Class Glow Plug Light On
Our customer brought in their Mercedes S-Class with the glow plug warning light coming on every time the engine was started. The light would come on and go off, then return around 10 seconds later for another 15 seconds before going out again. The car wasn't driving any differently, but the problem had been going on for about two weeks, so our Mercedes specialist technicians carried out a full diagnostic investigation.
Fault Codes
P2136 – Glow plug cylinder 4
Common Symptoms
- Glow plug warning light on when starting the engine
- Light going out, then returning after around 10 seconds
- Car otherwise driving normally
- Risk of harder starting, especially in cold weather
Mercedes S-Class Diagnostics
A fault code scan returned P2136, glow plug cylinder 4. The serial data showed no readings, there were no relevant technical service bulletins, and a visual check didn't reveal anything obvious. As the fault code was permanent, we unplugged the cylinder 4 glow plug and tested its resistance. It read 82 ohms, when it should be below 10, confirming the glow plug had failed. Replacing just that one glow plug would only have been a temporary fix, as the glow plug control module was shorting out and damaging the plugs. We explained to our customer that without a new module the fault was likely to return as the glow plugs burnt out, and they agreed to replace all four glow plugs and the control module.
Vehicles Affected
Mercedes S-Class
- Mercedes S-Class 2011
- Mercedes S-Class 2012
- Mercedes S-Class 2013
- Mercedes S-Class 2014
- Mercedes S-Class 2015
- Mercedes S-Class 2016
- Mercedes S-Class 2017
Mercedes E-Class
- Mercedes E-Class 2009
- Mercedes E-Class 2010
- Mercedes E-Class 2011
- Mercedes E-Class 2012
- Mercedes E-Class 2013
- Mercedes E-Class 2014
- Mercedes E-Class 2015
- Mercedes E-Class 2016
Mercedes C-Class
- Mercedes C-Class 2008
- Mercedes C-Class 2009
- Mercedes C-Class 2010
- Mercedes C-Class 2011
- Mercedes C-Class 2012
- Mercedes C-Class 2013
- Mercedes C-Class 2014
- Mercedes C-Class 2015
Glow Plug & Control Module Replacement
While removing the old glow plugs, one of them snapped, which can happen when glow plugs have seized in over time. We always take great care to avoid this, but it isn't always possible. We carefully drilled out the remaining section, then fitted all four new glow plugs and replaced the glow plug control module to stop the fault from coming back.
Resolution
After completing the repairs, we road tested the Mercedes, checked everything was working correctly and cleared the fault codes. The glow plug warning light stayed off, and our customer drove away confident the problem was fixed for good.
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