Error code P0128
Thread Starter
Newbie
Joined: Jun 2017
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E350 4matic
Error code P0128
I’ve had my check engine light for about 2 weeks now and diagnosed error code p0128. I installed a new thermostat and last night and still have the check engine light on. I drove it more than 30 miles already but doesn’t seem to go away. I tryed erasing the code but only comes back right away. My temperature shows 147 degrees Fahrenheit meaning it’s very low. I soon need to go for an emissions test but won’t pass with this problem. What should I do?
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Joined: Oct 2005
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How are you measuring the temp in Fahrenheit? The dash gauge is in Celsius. In any event I don’t believe your new thermostat is working properly. Was it an OEM part? They are not cheap. Your temperature is so low reflecting an open thermostat condition (failure). Your CEL will not go away until this is fixed. Once again, I think your new thermostat is bad.
Senior Member
Joined: Apr 2016
Posts: 457
Likes: 151
From: Phoenix, AZ
2016 E350 Sport 2WD
Go to Youtube and look for a video by South Main Auto Repair with title 'My Red Hondoo has a P0128 (and it's not the thermostat)'. Scan data shows car was warming up and all of a suddden the CTS dropped the temp reading drastically (temp didn't really drop as indicated by the CTS).
Thread Starter
Newbie
Joined: Jun 2017
Posts: 4
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E350 4matic
My OBD scanner gave me the reading in Fahrenheit. Converting it into celsius is about 67 degrees Celsius. It should be working properly since I did purchase OEM from the Fcpeuro.com
I don’t understand how it is low on my gauge it shows 95 degrees celsius for the coolant temperature.
How are you measuring the temp in Fahrenheit? The dash gauge is in Celsius. In any event I don’t believe your new thermostat is working properly. Was it an OEM part? They are not cheap. Your temperature is so low reflecting an open thermostat condition (failure). Your CEL will not go away until this is fixed. Once again, I think your new thermostat is bad.
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MBWorld Fanatic!
Joined: Jan 2009
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From: V E G A S
1922 Ford Model T / no OBD
I had several scanners with live data giving me false reading. TorquePro is telling me my exhaust DPF is reaching 1300 psi occasionally. 
I would confirm it with infra red to start.
And again, if you have electrovalve, it is computer that tells it when to open.
I would confirm it with infra red to start.
And again, if you have electrovalve, it is computer that tells it when to open.
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Member
Joined: Jun 2007
Posts: 119
Likes: 11
2010 GLK
Fabi139,
Not certain if you still have the code but I had a similar issue (it did go away eventually). I believe it is tied to the emission group so it's a bit stubborn. Here was my experience:
https://mbworld.org/forums/glk-class...orn-p0128.html
Not certain if you still have the code but I had a similar issue (it did go away eventually). I believe it is tied to the emission group so it's a bit stubborn. Here was my experience:
https://mbworld.org/forums/glk-class...orn-p0128.html
Newbie
Joined: Dec 2023
Posts: 1
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c180
My c180 gives this error nearly every month, i googled the p0128 code and it says about cat. converter, but my service man says that it can cause o2 sensor, so i changed the sensors but it still continues.. I think this is chronical problem of my car 
MBWorld Fanatic!
Joined: Jun 2012
Posts: 1,229
Likes: 188
From: Toronto
2022 E53 AMG
I have this same error and tried to change my own thermostat however it is a very involved task since on my engine you also have to remove the wiring harness. So my car is going to MB to change out the thermostat and pay to have this done as this one is over my head.
Anyways, you may have an issue related to the amount of coolant in the system and the only way to hack it is to do the following. Your system is "under" pressured and since there maybe air in the system, it's not allow the temperature to rise. One way to help with this is to fill the coolant without placing the full cap on the system and wait for the bubbles to emerge. This is dangerous but of course less if you already used a coolant machine which MB uses to charge up the coolant system.
Anyways, you may have an issue related to the amount of coolant in the system and the only way to hack it is to do the following. Your system is "under" pressured and since there maybe air in the system, it's not allow the temperature to rise. One way to help with this is to fill the coolant without placing the full cap on the system and wait for the bubbles to emerge. This is dangerous but of course less if you already used a coolant machine which MB uses to charge up the coolant system.
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Joined: Feb 2014
Posts: 6,190
Likes: 1,549
From: MA
2008 E350 4Matic, 2011 E350 4matic
My c180 gives this error nearly every month, i googled the p0128 code and it says about cat. converter, but my service man says that it can cause o2 sensor, so i changed the sensors but it still continues.. I think this is chronical problem of my car
