Class B Forums

Class B Forums (
-   General Discussion Forum (
-   -   DEF Warning Light (

Rok 03-01-2018 08:31 PM

DEF Warning Light
We just had our "Check DEF" warning light come on. I researched the internet to try to find what to do after calling and leaving a message with our dealership. What I found out there doesn't seem to jibe with what we've been told by our dealership, and the experiences we've had.

Here is what I posted on another forum (which should explain why I'm in a quandary):

I can't believe that nobody else has mentioned what our dealer told us. Maybe it is bogus, but it seems to fit with what experiences we've had.

Our dealer told us that there were two ways for the Check DEF Light to come on. The first was a mileage timer that is set for about every 10K miles. For some reason ours came on the first time at between 9K and 10K miles, not after 10K as you would expect if it was set initially at 10K miles. It came on again at just over 19K, and it just came on again at just over 29K (which corroborates what the dealership told us, but could be coincidence). This also is why I'm out on the internet researching the issue. Each of the two other times we had to take it in to the dealership to get the counter reset.

Other times we've got the warning from the other trigger: the DEF reservoir is actually low and a sensor tell us that. We get some DEF (in 2 1/2 gallon boxes) and put it in (there is a plastic jug inside the box). The problem is that the 2 1/2 gallons is just slightly too much and it overfills and spills and we have to rinse it each time because the fluid is supposed to be highly corrosive and we didn't want it to corrode stuff in our engine compartment.

It is such a pain to deal with that we have taken to just telling the dealership to go ahead and fill it each time we get an oil change--and we take the overpricing hit from doing that.

Anyway, since nobody else has reported a similar experience, I wonder if what we were told is all bogus. I just thought I would post our experiences here so others could comment.

Thanks. (Before I wrote this, I contacted our dealership and I am now awaiting their return contact.)


Oh, for the record, we have a 2013 Great West Legend SE Motor Home built on a 2012 Mercedes Benz 3500 Chassis with the 22' 9" length van body.

avanti 03-01-2018 09:01 PM

Anything is possible, but I have never heard of a mileage-based DEF warning. I see no reason not to believe that you are seeing level-driven alarms.

FWIW: A lot of people believe that it is better to keep the DEF topped off, rather than waiting for the alarm. There is a very-much-unproven theory that the alarm triggers bugs or awakens daemons in the rather fragile DEF system. I am not fully convinced of this theory, but, after having my one-and-only emissions issue a few miles after an alarm-driven DEF refill, I have moved over to the top-it-off camp, just in case. There is little downside.

Rok 03-01-2018 09:47 PM

Here is the message I just got from the (same) MB Dealership:

"DEF light should only come on when it is low on fluid.

However it can be triggered by a sensor that is malfunctioning. Or the heating element inside the DEF tank itself.

If a fill up does not clear the message / light.

Then it is sensor related."

I guess it is off to Wally World (Walmart) to get some DEF to spill on my engine.

Rok 03-01-2018 09:51 PM

Thanks, Avanti.

I've seen that elsewhere. Just a few minutes ago, someone on a different forum said that was the "new" consensus to keep it filled. Older threads also speculated that not letting the warning light come on caused problems because the computer wants to "see" that the DEF was being used.

I'll probably move over to the keep-er-filled camp after this, too.

Davydd 03-01-2018 09:56 PM

It comes on based on your driving. I expect mine to come on at around 8,000 miles driven give or take. If the DEF light comes on then I top it off. I also top it off before beginning any trips. I think it strange to overfill with a 2.5 gallon DEF. I've never had that happen after the light came on. Anyway, you are supposed to watch it and not overfill. What's left you can top off later. There are pumps at truck stops to top your DEF off. You can buy gallon jugs as well but a gallon never turns off the DEF warning. I think you have to top off. If you are getting service at a Mercedes Benz dealer they are suppose to top it off and I've never been charged. I thought that was pretty routine for servicing to check all your fluid reservoirs and top them off including windshield washer fluid. I guess I pick good dealers. The only one that didn't do that was a Freightliner dealer and I never went back. I've always driven Sprinters (three) still in warranty.

avanti 03-01-2018 10:27 PM


Originally Posted by Rok (Post 68471)
Just a few minutes ago, someone on a different forum said that was the "new" consensus to keep it filled.

I saw your post on the other forum. The "someone" who replied is on my "ignore" list, so I don't know what he said. But it is a significant overstatement to say that there is a "consensus". Lots of people wait for the light. As I said, though, I no longer do.

mlts22 03-05-2018 04:58 PM

I probably would have the DEF tank flushed as part of the "A" or "B" Mercedes service. This gets rid of the ammonia crystals, which often trip the "LOW DEF" warning.

Ideally, flushing it and topping it off is probably the best choice.

jon 03-05-2018 05:14 PM

I have had the light come on after the dealer topped off my DEF. Was caused by the technician not securing the DEF cap properly. I just had to retighten the cap at next stop. I keep a spare 1 gallon container of DEF fluid in the van just for emergencies. The one gallon size stores out of the way better than the large box.

Rok 03-21-2018 08:38 PM

We managed to fill the DEF without a massive spill this time. We did spill a few drops.
That left a bit in the box for topping off before our next trip.

Thanks to everyone who replied to this thread.

All times are GMT. The time now is 01:44 AM.

Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2020, vBulletin Solutions, Inc.