most common problems are a bad nox 2 sensor or a plugged def injector.

 

the way to tell if its the sensor is to put a reader on it drive it till its reading get it up to speed and let off and they should both get close to 0

 

my lml did this and had a miles countdown until it limited speed. i was driving 70 on the interstate when the countdown hit 0. it all reset itself and never came back on. you could tell him to try that

 

Moved to LML

 

Pull out the def injector and clean with water (it will dissolve any crusted def), it should be easy to do.

As others have posted, by bet is on one or both def sensors. A borderline sensor can drift in and out of 'ok', making troubleshooting difficult.

I'd change the def sensors before the ecm.

 

^ this, but it is not a DEF sensor, it is a NOx sensor.

 

Correct. There is no 'def' sensor on the truck.

Def is injected to reduce the level of NOx in the exhaust. The NOx sensors (there are 2) determine the effectiveness of the def injections.

 

My 2011 is doing same thing. Where do I find the NOx sensor and injector.

 

My 11 did the same thing (55k miles). I took it in, they told me it was because I used Blue DEF and not AC delco, they replaced the tank and all the lines (1800 bucks). The truck made it exactly 9 miles from the dealership and the light came back on, I took it back and they said the DEF injector was clogged, replaced it under warranty. The next weekend when I was 400 miles from the house, the light came back on, I was headed home luckily, the light came on, I drove a couple hundred miles and it popped up "55mph max speed upon restart" I kept driving with the intention of not shutting it off until I was home. About 20 minutes later everything went off and the truck was fine for a few weeks. It did come back on though and I took it in for a 3rd time, ended up being a NOx sensor the entire time. Haven't had any issues since.

 

one nox sensor is just after the turbo the the injector is just after the cat but before the dpf and the last nox sensor is in the dpf

 

can you replace a NOX sensor on your own without going thru a dealer or is there some calibration needed after changing one? thinking of just having one with me possibly. Or maybe just changing both sometime to get ahead of hte issues? I had the Service Exhaust fluid light 1 time for about 20 miles and it went out and hasn't been back on just yet. I always use BLue Def and I do check the Dates...

 

rox5488, did you have a 'def quality' message or just a 'service exhaust fluid' message?

 

It was the Service exhaust message. was on for about 20 miles or so then went back off. read the manual of course and talked about taking it to a dealer...

 

I have a 2012 duamax with a exhaust fluid quality poor message with no codes. While doing def quality test NOx 1 sensor reads 125 ppm and sensor 2 is reading 1.0 ppm. I have run the quality test and it ran over on time then I did a service regen with a quality test after twice with a ecm power down in-between and the message is still there. Any body have any solutions.

 

Another possibility is a stuck DEF gauge. My gauge was stuck, telling the truck it still had plenty in the tank when it actually ran empty. I got the service exhaust, 99 miles till reduced speed and service DEF system messages. I guess the dealers default first diagnosis is “contaminated” def. They told me that wasn’t covered under warranty. They pulled the tank to drain it and it was bone dry. They figured out it was the gauge, replaced it under warranty.