p20EE nox sensor readings

BigLou80

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Oct 7, 2016
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I've serached the forum a bit and come up empty handed. Truck keeps throwing a p20EE,
So far we have replaced both NOX sensors, the DEF injector and the ECM and the code won't go away.

While doing a deceleration test neither of the brand new NOX sensors would go below 200ppm even with a 0.0 fuel rate. I find it highly implauseable that both of the NOX sensors are no good. I live in a place where deleting isn't an option but I am $5,000 down the rabbit hole and not getting anywhere
 

2004LB7

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Dec 15, 2010
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have you forced a service regeneration? you may also need to run a DEF fluid reduction quality test. also new NOX sensors require the sensors to be calibrated after installation. just went through something similar on a family members truck and couldn't clear the codes until I did the service regen and reductant test. need a tech 2 or similarly capable tool

if the SCR is saturated with DEF it can cause false high NOX readings but a regen will clean it out.
 

BigLou80

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Oct 7, 2016
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have you forced a service regeneration? you may also need to run a DEF fluid reduction quality test. also new NOX sensors require the sensors to be calibrated after installation. just went through something similar on a family members truck and couldn't clear the codes until I did the service regen and reductant test. need a tech 2 or similarly capable tool

if the SCR is saturated with DEF it can cause false high NOX readings but a regen will clean it out.
been thorugh 3 regens since the ECM was reaplaced less than 50 miles ago, both the nox sensors were reset and the front one isn't subject to ammonia loading like the rear one but both are reading 200 PPM with no fuel to the engine.
 

2004LB7

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I was seeing around 200 ppm on the upstream sensor and I think about 20 to 50 on the downstream. after the regen was finished. maybe your aftermarket downstream sensor is bad. have you tried cleaning the old sensor and seeing if it works?
 

BigLou80

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Oct 7, 2016
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So you haven't ran a reluctant quality test? You will need to as a last step after all you have done.

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no one was able to run the test... so the ECM was replaced and the NOX data was no good and predictably as soon as the new ECM complted a drive cycle it trigger the low DEF quailty and started the derate countdown. Seems to me untill the sensors are reading as the should there is no sense in running a dedicated monitor cycle that destined to fail unless the test somehow recalibrates the sensors
 

BigLou80

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Oct 7, 2016
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I was seeing around 200 ppm on the upstream sensor and I think about 20 to 50 on the downstream. after the regen was finished. maybe your aftermarket downstream sensor is bad. have you tried cleaning the old sensor and seeing if it works?
neither sensor will go to below 200 PPM even when doing a Deceleration test. when according the GM's diagnostic with fuel cut to the engine while you coast down there should be no NOX at either sensor both of these drop to 200 and stop. The scanner says fuel has been cut with a 0.00MM3 fuel rate and all threee injection states ( pre, main and post) reading as off

I just can't wrap my mind around 2 brand new NOX sesnors going bad in less than 20 miles
 

KyleC4

Tech
Dec 30, 2016
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Main reason for them asking you if you’ve ran a reductant quality test is so that you could see the data of all the other sensors during the test. Cause Nox 1 is right out of engine. If that doesn’t drop below 200 on coast down then start looking at other possible problems. I’m curious what the quality of fuel is. API rating? Contaminated with anything? It would also be suspicious to me if the scan tool being used is only dropping to 200ppm and not reporting actual data. I guess it’s possible that injector(s) are leaking and even when you tip out of throttle to coast down some fuel is still getting into combustion chamber. Or I guess exhaust could be plugged alittle too. Too many variables here. I’d like to see a reductant quality test so I can look at all egt sensors, nox sensors, maf sensor, def command, indirect command, etc. I’m just rambling to hopefully give more ideas on what to look for
 
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BigLou80

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Oct 7, 2016
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Main reason for them asking you if you’ve ran a reductant quality test is so that you could see the data of all the other sensors during the test. Cause Nox 1 is right out of engine. If that doesn’t drop below 200 on coast down then start looking at other possible problems. I’m curious what the quality of fuel is. API rating? Contaminated with anything? It would also be suspicious to me if the scan tool being used is only dropping to 200ppm and not reporting actual data. I guess it’s possible that injector(s) are leaking and even when you tip out of throttle to coast down some fuel is still getting into combustion chamber. Or I guess exhaust could be plugged alittle too. Too many variables here. I’d like to see a reductant quality test so I can look at all egt sensors, nox sensors, maf sensor, def command, indirect command, etc. I’m just rambling to hopefully give more ideas on what to look for
I will run one, but the EGT sensors are all within what I have observed to be a typical range, I dont have the exact numbers in front of me but everything looked as it should. The fuel is just good ol speedway diesel that i've used probably 100 times before but it's been a while since I used this station, truck only had 2 gallons in the tank after dealer installed ECM and ran 3 regens so it was that or nothing. All the balance rates are under +/- 2 at idle so while a leaking injector is a possiblity I would expect to see that reflected by the balance rates prior to this all starting by replacing the DEF tank heater the truck ran fine, got reasonable fuel mileage and up to 768 miles before starting a regen. Would also expect a leaking injector to show with shorter regen intervals As for the other data values I don't have DEF injector command or MAF readings handy

The values bounce all over when driving #1 shoot to almost 1200 while accelerating ( probably what triggered my P2BAD code) and both are in the 300's when crusing. the lowest reading was 193 on the front and 204 on the back during the deceleration but they both liked to hover in the low 200's for the 3 times I tried this. the rear typically reads higher than the front..... which is also suspect. I will borrow another scanner and see if it reads below 193 PPM. Maybe it's an issue with my scanner

Thank you for replying
 
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BigLou80

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Oct 7, 2016
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^Exactly.

Fuel, oil, coolant, DEF, all can be reported as NOX by the sensors. I've chased my tail diagnosing NOX sensors on a truck that had an inaudible boost leak.

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I should run a boot leak test but when looking at the leak ratio on the last RFQT I ran it was1 :.98-1 for the reported leak ratio for what that's worth

OUA hasn't shown any coolant contamination and the only time I have ever needed to add coolant was last week after changing the thermostats to eliminate the engine not coming to temp as a possible reason for the RFQT to fail
 

BigLou80

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Oct 7, 2016
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For what it's worth after installing a new DEF injector and both NOX sensors everything read as it should 300's on the front and <70 on the rear which is a 70% reduction . WE thought we had the problem solved and tried to run an RFQT that failed and eventually the computer threw a P0606 got that code out by changing both batteries but from there on out the truck was stuck trying to regen with not enough miles to drive above 4mph on the countdown to actually do a regen. since regen inhibits the DEF injector there was no hope for ever completing a RFQT and neither my scanner, my buddies scanner nor the dealerships scanner could overide the Regen and all three were struggling to stay connected to the ECM.

Dealership replaced ECM, ran 3 regens and said drive the truck. I looked at the NOX readings as soon as I picked up the truck and knew it was only a matter of completing the drive cycle to trigger another derate and the DEF quality low message and sure enough after 20 miles and as soon as I accelerated to highyway speeds on comes the CEL and the LOWquality DEF
 
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2004LB7

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you need to run the reductant quality test. you may need to run it several times as it's not unusual for the first two to fail. if it fails three times in a row then start looking at the codes and inspect everything to make sure it's working as intended. without the test completed the ECM will not run all the equipment as normal and the readings can be off
 
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KyleC4

Tech
Dec 30, 2016
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And did I read all your posts correctly…….? Did this all start after replacing the DEF reservoir ? I’d immediately think bad DEF fluid got put in it. So my best idea here would be to run a regen. Look for for egt sensors to read egt 1 = 700, egt 2 = 1000-1200, egt 3 = 1000-1200, egt 4 = 1000. Those are approximate values. This will tell you that your SCR gets hot enough to help reduce NoX and the 9th indirect injector is working. It will also help burn out any DEF that may have already saturated the sensor. Also look at what your ppm values are. After that test run a RFQT test. And closely watch the NoX readings and DEF command. If NoX 2 shoots up once DEF starts to be injected then there’s a problem.

Keep posting with your results. You have me curious on this one
 

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