Ever get derated to 5 mph because of no DEF when your def tank is full?

You're not out of DEF. The flashing light on the DEF gauge means you have an emissions fault. Could be DEF quality, contamination, bad NOx sensor, or fifty other things.
The code set will give more information.

 

When their is a problem with the DEF system or sensors. That what happens. Fist your DEF light will flash. Then check engine light comes on. If driver keep driving or does not notice. I think the system is set let truck go 500 miles. If not fixed then the red engine light comes on and truck will limit speed to 5MPH. It could be bad sensor or low nox conversion. Something is messed up. They will need a computer to fix it and see what the computers are saying is wrong.

 

Try doing a parked Regen. If it's able to do one it may clear the 5 mph limit for you.

 

That sounds like your DPF maybe clogged. If you do a parked regen and it dont stay in it longer than 15 or 20 min, then your DPF is clogged. What type of truck do you have? What year?

 

You could go a little faster than 5 mph if you take speed sensor off.

 

I had 2011 truck had same problem back then. Freightliner always did sames test to see what was wrong. First they would check DEF quality. Then they would test DEP pump output. They had to measure it in a cup. Then they check the Nox sensors. Mine was the Nox sensors. Then they would check get truck check for codes, if none send the way as fixed.

Then I had same code come back low Nox conversion. They did the same checks again. Since it was all good. Then they said it emissions box was bad on Freightliner with Detroit engine. The One Box. Hopefully it just a sensor. I don't think TA will do that kinda work on side of road even if it just sensor. Your probably have to get a tow truck. Hopefully not.

 

I remember one time my "check engine" light came on,, so i opened the hood and the engine was still there, so i guess a sensor was bad.