NOx Sensor Readings

[JReichenbach]

Hi all, picked up a 2012 3500 cab and chassis for a buddies Hydra-Bed. Def equipped, 4x4, auto, 161,000 miles on it. Had a code for invalid communication coming from front NOx sensor when bought. It appeared that someone had already replaced the sensor assembly, no names or part numbers on it. First off, tried cleaning both sensors by soaking them overnight in injector cleaner, no help, code came back. Checked wiring connections and ohmed circuits to make sure everything was hooked up. Still got code. Finally ordered a replacement Standard Products sensor and installed and test drove it 15-20 miles, no code. Took the truck on a 70 mile trip to get bed installed, and check engine lite came on about 50 miles out. This time it says Invalid Communication from rear NOx sensor. cleared it, good for roughly another 50 miles. My buddy ended up taking the truck on a 1600 miles trip to tow a feed wagon back, so it had somewhat of a load on it. He also had been feeding cows with it, and racked up about 3000 miles on it before I got it back. He stated that truck ran great the whole time, and had been using DEF, so I'm assuming it had been regenerating. I had just went ahead and gotten a new Standard products sensor for the truck.
Now, up until this point, I had not used a scan tool to monitor the Nox sensor readings while driving truck. Changed rear sensor, hooked up scan tool, and went for a drive. At first the D/S NOx sensor showed 65532 PPM, while U/S was anywhere from 200 to 500 PPM, depending on where my foot was into throttle. After driving for a bit, the D/S starting reading approximately half of what the U/S sensor showed. When I would lift my foot off of gas pedal a lot, eventually the D/S reading would then go back to 65,532 PPM. When I started to accelerate, or go to a steady cruise, the D/S numbers would come back to half again. A couple of times, the U/S number would jump to 65,532 PPM number, but rarely, and no pattern seemed to be involved. Drove truck approximately 20 to 30 miles this way, check engine lite stayed off.
Any ideas what is going on here, I'm more of a gasoline mechanic than diesel, so this has me puzzled. Apologies for the long post, but tried to get in everything I could. Any help would be greatly appreciated. Thanks, Jim.

 

[Signal73]

I didn’t see what code you scanned but check out this info before going much further:

https://static.nhtsa.gov/odi/tsbs/2014/SB-10064533-0335.pdf

 

[AH64ID]

DEF does deal with NOx reduction, but DEF isn’t used for regeneration.

It is possible that the ppm reading goes off the chart while coating because they cool down. I recall NOx sensors having to be at a certain temp to operate.

 

[sag2]

You need to test the Nox sensors on a closed throttle downhill. Coasting with throttle closed, the NOx should be zero or very close to it. At that time the engine is only pumping air through the engine, so the NOx should not be there unless there is a fuel (or any hydrocarbon, ie diesel or oil) being burned when the throttle is closed. When on the throttle or at cruise, the upstream NOx should be higher than the downstream, assuming the DEF injector is currently injecting fluid into the SCR CAT.

 

[sag2]

FYI, there is no specific number to look for NOx. It is zero on closed throttle decel, and upstream low to very high at cruise depending on load, ambient, and exhaust temps. If the SCR CAT is working properly downstream is lower than upstream.

 

[JReichenbach]

Thanks for all the info, I will get truck back and check the tips out. Was wondering if bad Def fluid could be part of problem, as truck had been sitting a while before I picked it up. I'll have to go out and look at my notes for the codes that were showing. The tip about sensor temperature does seem to factor in. When truck was first started, the 65532 reading comes up a lot more. While cruising down the hiway, the sensors work as described by sag2, I'll have to try the coast test again, but usually the D/S sensor goes up to 65,532 when slowing down. Weather was in mid 20's here when I was test driving it.

 

[JReichenbach]

Well, he drove it for a few days, roughly 100 miles, the CEL came on. next nite he called and said the message center was showing the DEF system needed dealer service in 193 miles. Pulled P207F and P20EE codes. He had not added any Def fluid to it since I looked at it last. I did look at the TSB that Signal73 mentioned, but it does not apply to cab and chassis trucks.
Time to look at def injector???