Cf 75 exhaust fault

Gents I have a 12/14 build cf 75 which is coming up with a exhaust system fault then 5 minutes later the engine power limiting.
Codes that come up are 121 and 128 they are NOX concentration codes.
I have drained adblue tank and refilled,changed all adblue filters and ran test procedures for pump pressures air and fluid and electrical conections.
We changed the dosing valve and ran a metering test which all came back correct.
I’ve read about taking it for a hard drive and graphing information but no one here in aus seems to know anything about or willing to do this.
Are there any more tests we can do to help diagnose this any help appreciated.

If you are sure the ad -blue side of the system is working ,then you need to do a NOX monitor test ,which you will need a DAVIE for ,there is a dynamic test that will test the CAT and a static test for the NOX sensors the results are translated in to a graph which should show what is at fault ,generally it is a NOX sensor that isn’t reading right …

thanks but there’s that word that no one at the dealer has in their vocabulary (graph,not just my experience). Are the NOX sensors before and after interchangeable

Afraid you cant swap them over …The dealer should be able to do the test ,though I am the only one at our place who can do it and understand it …If they can manage the tests they can send the results to technical for help …Instead of buying two NOX sensors separately at whatever the cost ,they do the 2 of them together cheaper than buying them separately ,but it is a different part number …Again you may get a confused look of the parts

thanks for putting up with me dont have much faith in our only dealer last guy drove in with similar fault stayed for a week got charged an enormous amount of money just to drive out in limp mode.
I think my next test is to remove engine pipe and check atomization of adblue (could this cause problems is it unheard of i assume it should be a fine mist) even though pressures are ok.
Just for some more info fault seems to be brought on just after truck has climbed a big hill or done some 100km/h freeway driving

d3bert:
thanks for putting up with me dont have much faith in our only dealer last guy drove in with similar fault stayed for a week got charged an enormous amount of money just to drive out in limp mode.
I think my next test is to remove engine pipe and check atomization of adblue (could this cause problems is it unheard of i assume it should be a fine mist) even though pressures are ok.
Just for some more info fault seems to be brought on just after truck has climbed a big hill or done some 100km/h freeway driving

That is ok …First my stamping of my feet lol…The system has been out long enough that no dealer should have a problem diagnosing it ,unless they have a high turnover of staff or don’t a beep…

The 2 codes you have are classed as group faults and I think they are also long term faults that will stay in the ecu for about a year as long as what caused them doesn’t reappear then they vanish …Unfortunately there is about 100 individual faults that can bring those 2 codes on ,generally you would get another normal code to go with them

The way the system works in a nutshell …The vehicle will not use ad-blue until certain parameters are met ie temperature before and after cat,and the vehicle has to be underload ie going up a hill fully freighted …Nox is only created when the vehicle is underload and when all the other parameters are met then ad-blue is injected in to the exhaust stream hence you get your warning when going up a hill…When descending the engine isn’t doing much so no ad-blue is used

Have you checked for a new code since the last time as you may now have a code that points to the fault

Generally if you remove the exhaust pipe at the silencer you can check for bi=uret build up on the face of the cat ,it looks like snow…On older vehicles check the actual dosing valve as the can corrode away and ad-blue will leak out …There are tests in DAVIE that we do to make sure it is dosing the correct amount ,like everything nowdays you need a computer

thanks for your patience, just an update.
Back at the dealer today redid all the the static tests again all passed as always (pressures, metering).
fault 121 was not present but replaced with 127, 128 still present all in active, 127 is another group fault.
techs had heard of testing truck on a hill but never done it so not really interested, we did come to the conclusion that I will go back and take a tech for a drive and try and get it to fault with him monitoring davie hopefully get to do this in the next few days as this sounds more productive than reseting and hoping it logs a fault next time.
thanks again

If a unit it needs to go out with a loaded trailer ,if a rigid it has to be also loaded …Take it out ,get all the exhaust temps above 200 degrees ,when back at the workshop they have to if they have an old DAVIE XD go to regulation functions ,overview and read the NOX sensor PPM, if using the new DAVIE more likely go to monitoring and look at the NOX PPM and record it ,now using your controls set the revs to 1200rpm and just sit there for 5 mins ,there is a different parameter depending if a drum silencer or a stack …Now you are just sat there at a fast idle so you are checking the NOX sensors only so the reading in and out should be pretty consistant ie 200 ppm moving up and down slightly if any reading is erratic and all over the place that sensor is faulty and any reading which is I think above 220 ppm then again that sensor is faulty.He should be able to get the exact figure as there are bulletins

fellas my woes continue after last reset drove it like nanna going to church and it lasted nearly two weeks before faulting (might have been in a hurry to get home that day)., back to dealer reset and off for a drive (I did tell them about the static nox test). it faulted on freeway noticed before cat nox sensor reading erratically when it faulted. Back and change nox sensor. drove local next few days all good next day on the freeway after 15 minutes fault again.
Temp gauge on the way to freeway was consistant approx 1/4 (75 degrees i think) prior to faulting it was just under half on the gauge. Gauge does fluctuate quite regularly under different conditions.
Is there anything else to look for (I know you guys dont have a crystal ball) maybe fuel pressure or turbo boost. noticed on davie it had a low fuel pressure fault 5 months ago inactive. I remember this didn’t know what it was and it went away after 15 or 20 minutes. I have a good indy with a jaltest that wants to help but we are a bit lost.

sent a PM