I am interested in cars and do my own repairs where I can. I like to share any knowledge I have to help other people where possible.
Possible Signs of a Bad DPF Sensor
Is your DPF (Diesel Particulate Filter) warning light on? Is your car in limp mode?
The diesel particulate filter (DPF) is a part of the exhaust system, designed to filter out particulates and reduce pollution. The way it is supposed to work is that it collects soot and ash; then, on long journeys, the car raises the exhaust temperature, performs a cleaning cycle and burns it off.
One problem some people have with their cars—and this is quite common with the Mercedes models in the title—is that they're only running around locally and don't go on long journeys, so the car never gets the chance to run the cleaning cycle, the DPF fills up and the car then puts itself into limp mode. Another common but distinct problem is that the pressure sensor fails, which makes the car think the DPF is full. In either case, the outcome is the same: limp mode, reduced power and a warning light on the dashboard.
If the DPF really is full, you can force the regeneration of the DPF which can solve the problem. If this doesn’t work because the car says the DPF is too full to regenerate you then may have to replace the DPF.
But before before doing that, check that it isn’t the sensor, not the filter, and replace the sensor if necessary. In fact it is so cheap to do, that you can do that to rule it out with little outlay, and in my experience doing that can sort out the issue. In fact my car has been through two DPF pressure sensors since I bought it a couple of years ago, so I thought I'd write a guide on how to replace it as I struggled to find any info online.
Apart from anything else, simply finding the location of the DPF sensor took some doing, which is why I’m sharing this info to help you to find it quickly.
To do this job properly, you'll need to not only replace the sensor, but you'll need to delete the fault code as well. To do this you'll need a diagnostic machine. The one I use is an icarsoft MB v2.0. These are cheap and very useful for diagnosing problems. If you buy one for this job it will cost you about the same as it would to take your car to a garage and get them to do it, but once you have it you can use it again and again so I think it is a useful and worthwhile investment.
If you plug the diagnostic machine in and read the DPF values while the engine is running, if the sensor has failed (in my experience anyway) it will tell you that the DPF is 3-400% full and a regeneration isn't possible. At this point, you can try a new sensor to hopefully remedy the issue.
How to Read Engine Data About the DPF
If you look at the pictures below, you can see how to read the live data from the engine, to see the state of the DPF.
This is the device you need to delete the fault code and get the car working properly again.
First select "Diagnostics."
Then select "Manual."
Then select the ECM module section: the first one on the list.
Go to "View Data," "Select All: and with engine running, scroll through til you see the DPF fill level. If the device shows the DPF 3-400% full, it is actually probably a sensor fault.
Tools You'll Need
Apart from a diagnostic machine, these are the tools you'll need to change the sensor. An E11 torx socket and a pair of long-nose pliers.
How to Find the DPF Sensor
The OM651 engine is used in most Mercedes diesel vehicles on the road: the C class and E class, plus the Sprinter and Vito vans. The engine is in different states of tune in the different vehicles, but basically it is a strong engine, good for many miles.
In the car the DPF sensor is located behind the engine block at the back next to the bulkhead. Although just out of sight, it is pretty close to the top of the engine, so it is easy to reach and replace. I have tried to illustrate the location in the photo below.
This is where you'll find the DPF sensor. You'll need to remove the engine cover though to get access.
How to Remove the DPF Sensor
The sensor has two rubber hoses going to it, with an electrical connector. The hoses are connected to the exhaust before and after the DPF. The idea is that the sensor measures the pressure differential between the two points, to determine how full the DPF is, so the car knows when to trigger a regeneration cycle. When the sensor fails, the DPF is then falsely read by the system as full to the brim, and as the car then goes into limp mode, the regeneration cycle won’t run.
To get to the sensor you firstly have to make sure the engine is cool, then reach your hand down the back of the block just behind the dipstick. The sensor is about the size of a matchbox, but easy to find. It has two torx bolts, one that fastens it in place and the other that fastens the two pipes. If you undo both bolts, then you can get access to remove the electrical connector. This has a pushdown clip on it, but other than pushing down the clip you simply pull it off.
To remove the hoses, firstly you need to detach the rubber hose clips. This is done quite easily with your pliers. Then you can simply put the end of the long nose pliers into the bolt hole on the sensor and while holding the hoses with your hand, pull the sensor up using the pliers for extra grip to detach it from the pipes. It pops off quite easily. You can then fit the new sensor by holding the pipes one at a time with the long nose pliers, while pushing the sensor down onto them. They are different diameters so you can't mix them up. Then reattach the hose clips to secure the hoses and reconnect the electrical connector. The sensor then bolts back into place.
This is a difficult process to demonstrate as there is very little room where the sensor lives to get a camera, but when you lean over the back of the engine and look down there you will see the sensor straightaway. If you have long enough pliers, it is pretty easy to disconnect and reconnect the pipes and connector. Just don't drop your ratchet while tightening the bolts!
How to Delete the Fault Code after Replacing the Sensor
After replacing the sensor you will need to delete the fault code to get the car to run properly. Follow the same stages as above with the diagnostic machine, but instead of viewing data, select "Clear Fault Memory."
Find Out if You Fixed the Problem
After this you can then start the car and go for a drive. If the fault was the pressure sensor, then you'll be back to full power and as you've cleared the code the engine light will be off. Rejoice and drive around with a great feeling of satisfaction!
I have found that the sensor only lasts a few months on my car, then it goes again for some reason, but as they're not expensive it is a bit of an inconvenience but not too bad to replace the sensor yourself.
Of course if the DPF is full to the brim and replacing the sensor doesn't do the job, then you have a different job to do. If that happens to me, then I'll write another article documenting it, but so far replacing the sensor has worked both times for me. Hopefully it will solve your DPF issue too!
I hope this guide was useful. I am not a mechanic, but I have an interest in fixing my cars where I can. If replacing the sensor doesn't work for you then I'm sorry, but if it does then you have fixed the car for very little money and not a lot of time.
Thanks for reading.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
pickles666 on June 12, 2020:
if your finding access difficult you need to remove the wiring loom plug from the ecu on top of the airbox, remove the front turbo pipe and surrounding inlet pipes ( the huge ones that feed air into the airbox ) unclip the wiring loom harness from the airbox and push to 1 side, the airbox will now lift out, its on 2 rubber bungs at the rear and simply pulls off. you will now have all the room you need to work in.
RickDangerous on June 11, 2020:
I got as far as the clips off, but couldn't shift the pipes.. with so little room in that space, i was worried if I managed to get them off i wouldn't be able to reconnect them... any tips for stuck on pipes?
Mikey B on June 09, 2020:
This is brilliant! To clear the fault codes I’ve been using the Car Scanner iPhone app connected to a WiFi OBDii reader that I picked up off amazon for about a tenner. This would usually be enough to allow a motorway-drive regen to occur, but it recently started also showing dpf circuit and sensor failures and so won’t go out of limp mode any more. I’ve been trying to find the actual sensor location for 3 years! Hopefully I’ll be able to change it now. Many thanks!
aRtie on March 02, 2020:
Thanks a lot. This helped me recently with our C200. We had to perform the manual regen, but since then, the car drives as before. Really appreciate your guide.
Prem Naidu on January 01, 2020:
Where can find knock sensor 2 (cylinder bank 1)
Ray on December 12, 2019:
My vehicle is stating the same issues, if this works I will re-post
James on December 02, 2019:
Your advice was spot on, my DPF showed 400% and my car was in limp mode, after crazy quotes l decided to take on the tricky and painful task of replacing the diff pressure sensor. lt was very tight and my hands are sore. After installing and a short drive my DPF dropped to 150% and limp mode went away and after a 30 min run on a highway the DPF fully regenerated and the DPF went to 0%. l used my icarsoft MB V2.0 bought on ebay to monitor the live DPF regeneration as l drove with my daughter. Based on my experience the advice to replace this sensor at about 8 years old is correct because it seems to have caused my DPF to stop regenerating and the car went to limp mode.
Marcus on November 13, 2019:
Thanks a lot for this guide! really useful and has helped me recently with my E Class C220.