Quote:
Originally Posted by ard
Which (I think) is why a 'bad urea' warning can be due to a clogged DEF nozzle: the duty cycle goes to its maximum limit; the DDE does not see the NOX values respond as they should; it then declares 'must not be def in the tank'
|
You get the idea, but there's an additional complication that doesn't allow a diagnosis by "maximum duty cycle"
The NOx sensor is not capable of distinguishing NO and NO2 (the NOx) from NH3 (ammonia, which is what the urea converts into once it hits the hot exhaust gas).
There is a phenomenon called "ammonia slip" that can occur when too much urea is injected. If there isn't enough NOx in the gas stream than some of the ammonia has nothing to react with and is emitted through the tailpipe which is something you don't want.
So the DDE changes the duty cycle to minimize the reading it gets from the NOx. A "high" reading could mean both too little urea (some NOx passes the SCR) as well as too much urea (some NH3 has slipped) and iteratively tries to target the low point. This is registered in an adaptation value to compensate for various tolerances in the system. The quality of the fluid is diagnosed by observing how marginal changes in the duty cycle affect the marginal changes in the NOx sensor reading, but also as sudden (meaning above a certain threshold) changes between the closed loop readings (the action-reaction of duty cycle to NOx sensor readings) and the adaptation tables.