P0420 - Bad NOX Sensors?

**druff** · 1 Week Ago

So I was towing my race trailer yesterday, and on the way home, I got a CEL for P0420, and it's the only code.

I noticed something weird on the way to the track and back.

The EGR valve is in a constant state of open, and im building a soot level in my DPF, which is usually at 0.

Trucks are also running hot coolant and EGT-wise, all things are pointing towards this poor girl running incredibly rich while pulling the 10,000-lb sail, which is my trailer.

We came to a stop light, and the EGR valve shut to 0, but the throttle also pinched off to 15%.

We took off from said light, and the EGR stayed shut, and the throttle opened back up.

The truck ran great for 10 miles before it opened the EGR valve again and started with its crap.

We went from 195 coolant temperature and 0 grams in the DPF after, and we burned out the soot to 208-212 coolant temps and built a soot level.

Now, I've seen nox sensors fail in a fashion where they say the emissions output is X, but the analyzer is saying it's way richer.

So according to the sensor, everything is good, but the engines are running way rich, but thats also on 2000 HP natural gas engines.

Everything to me is saying this truck is running rich, or the ECM is receiving bad feedback thats saying it's lean and is trying to richen it up; the cats will have issues when they are just getting hammered with fuel and soot.

So, does anybody know which nox sensor controls the engine's AFR?

Is it the one right behind the turbo?

I just put a new turbo on the truck about a week ago, but it did something similar before when towing (like a year and a half ago), although it didn't trip a P0420 light.

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**AK Bob** · 1 Week Ago

See the attached troubleshooting information for the P0420 code.

The over-fueling condition most likely causes the code.

I am not sure if the NOX sensor would cause this without a NOX trouble code.

Hopefully, others will chime in.

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**druff** · 1 Week Ago

To anyone following this saga! Ok I had it backwards, EGR lowers nox. Trucks saying hey im rich lean me out. I was thinking EGR raised nox and lowered combustion temps. Got that sorted!

Ok asked myself this, whys my truck running rich? Were on OEM tune with a stealth module. She cannot be pounding that much fuel.

Download alfaobd because ezlynk is stupid and wont let you look at the nox sensors and i just happen to have an obd adapter. Aftertreatment inlet is reading normally. No wild spikes that would raise cause for a bad sensor. Spikes as you lean into the throttle and leans itself back out. Completely normal stuff. Aftertreatment outlet reading a whopping 5ppm and is consistent there. With the occasion bump to 7 when the inlet was reading 300. (I floored it i wanted to be sure) obviously the SCR is doing its thing.

Obd adapter starts acting dumb cause its lord knows how old, plug ez lynk back in. Notice something and remembered a trend from my industrial stuff and racing. Alot of turbos max rpm is like 100,000 or something in that general range. Bigger turbos max out at lower rpms cause duh.

Notice my turbo rpm is 48,000, damn near half of rated rpm to make an entire 3.8 psi of boost at 1600 rpm or whatever 70mph is. Ight, thats weird. Flip a ***** on the highway, floor it, 90,000 rpm+ like 23 psi. Way off the 35 the trucks rated for and the 40 that the turbo is tested at

Boost leak. A fricking boost leak. Ya know whatll make an engine have high nox and make the ECM open the EGR to lean it out? A boost leak. I tightened both boots on the intercooler, the turbo to cooler boot looked suspect. If it persist, ima just buy a boost leak tool.

How is it, these trucks are so sensitive, that the smallest exhaust leak will trip a CEL but it missing over 10 psi of boost and its like "nah youre good!"

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**AK Bob** · 1 Week Ago

Thanks for the great write-up and for sharing your experience.

All makes sense now with the boost leak causing the rich condition.

The OBDLink MX+ dongle works very well with the AlfaOBD software.

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