Looking for some help with the dreaded P20EE code - SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 1. 2017 F350 6.7L fully stock with about 123k miles on it. Just started throwing this code within the last 2 months but only when not towing, towed 13k lb camper over 1600 miles a week after it first threw the code and it did not throw the code ever when towing but did when not towing about 3 times over 400 miles. No other symptoms, everything seems fine with MPG and no black exhaust or anything, regens seem to be occurring at the same intervals that I am used to seeing. It is listed as permanent code but the CEL will turn off if I "clear" it. DEF concentration % is varying between 31 to 32.8% I am using OBDLinkMx+ to monitor and run diagnostics along with OBDLINK app and Forscan lite. I have some freezeframe data which I will list here. Should I just replace the NOx sensor? Leaving on a trip from MN to Florida in 5 weeks pulling the camper. Not too worried about it but the wife is... Thanks for any help you can give.

Freeze Frame Data:
Code: P20EE - SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 1

Status:
- DTC Present at Time of Request
- Malfunction Indicator Lamp is Off for this DTC

Module: Powertrain Control Module

Freeze Frame #1:
-EVENT_TIME: 9768149 s (Sat Nov 9 10:51:12 2024) - Event time
-TOTAL_DISTANCE: 126713 miles - Total Distance
-LOAD: 45.49 % - Calculated Load Value
-ECT: 192 °F - Engine coolant temperature
-RPM: 1993 1/min - Engine Revolutions Per Minute
-MAP: 20.6 psi - Manifold absolute pressure sensor
-VSS: 41.0 mph - Vehicle Speed
-IAT: 43 °F - Intake Air Temperature
-MAF: 15.14 lb/min - Mass Air Flow
-PTO: Off - Power Take Off
-RUNTM: 1580 s - Time Since Engine Start
-BARO: 14.1 psi - Barometric pressure
-VPWR: 14.70 V - Control Module Voltage
-LOAD_ABS: 120.00 % - Absolute Load Value
-AAT: 43 °F - Ambient Air Temperature
-APP_D: 35.69 % - Accelerator Pedal Position D
-APP_E: 35.69 % - Accelerator Pedal Position E
-FUEL_TYPE: DSL - Type of fuel currently being utilized by the vehicle
-APP_REL: 24.71 % - Accelerator Pedal Position Relative
-EOT: 192 °F - Engine Oil Temperature
-FUEL_TIMING: -0.62 ° - Main injection timing relative to top dead center
-FUEL_RATE: 0.45 L/h - Engine Fuel Rate
-TQ_DD: 42.00 % - Requested torque output of the engine by driver
-TQ_ACT: 42.00 % - Calculated torque output of the engine
-GP_LMP: Off - Glow plug lamp
-PTO_STATUS: Off - Power Takeoff Active Status

 

I don’t see any nox sensor data .


are you injecting def?

the box sensors are a PIA to remove so if you go that route be prepared for some work.


i had to cut out the last nox sensor I replaced

 

Some pics

ox is second row from the bottom

nox1 and nox2

from cold at 45 degrees to a pic every minute there after

note that in the second pic the egr valve opens and NOX immediately drops

 

That freezeframe is just copy/paste from Forscan Lite, I am not very familiar with how to read the data or if it would be helpful or not. It is strange there is no NOx info in it since it is from that code.

I am not sure what you mean when you ask if I am injecting DEF. It is a stock system and does use DEF and I can see the DEF level is going down so yes DEF is being injected, is that what you mean?

I will have to see if OBDLink has the NOX readouts available and add them to see what I am getting, thank you for the reference readings.

 

my question is can you tell is you are injecting def ...some scan tools can log def injection


there are a lot of moving parts to NOX problems SCR, DEF system, EGR system, NOX1 and NOX2

I wouldnt jump into changing a NOX sensor unless you have clear data to support a nox sensor problem.

for example, for NOX1 note the pics I posted on NOX1 behavior during the first 5 minutes after a cold start.

nox2 will follow once the exhuast system warms up.

 

Ah I see, I don't think I have the ability with the OBD reader I have to log that. I wonder if Forscan on my laptop can do that. I will do some research and see if I can get a log of the DEF injection.

If I can get the NOx sensor readouts with what I have I will check that on a cold start and post the readouts from a cold start.

Thanks, I had no idea even where to start with diagnosing this.

 

I have Idash, forscan, and odblink

odblink is free with the MX adapter

I use odblink if I am in real trouble

odblink seems to see more than idash and forscan

 

I am using OBDLINK MX as well with the OBDLINK App. Here is what I get with the cold start, I had to go drive a bit to warm things up and get the EGR to start opening up. Not sure what these mean and no code at this time, maybe I just need to be monitoring while driving and if it throws the code I can see what the sensors are doing then? I also found the log page and added DEF Injection Command, I don't recall the actual PID listed and no idea if this is the correct one to log. Only ran the truck for about 20min and mostly parked, the log was flat line. Seems normal to me since DEF in my experience is barely used.

 

My nox photos were about 1 minute apart .

once warmed up, my nox2 is always lower than nox1


your nox2 is always higher than nox1

if your nox1 and Nox2 were the same…I would be thinking scr or def system….but your nox2 is higher than nox1 …more nox at the end of the tail pipe then at the beginning of the pipe…I think you found your answer.

maybe others will chime in on other possibilities .

are you thinking of fixing this yourself? Or taking it to a shop?

A few nox2 replacements on the forum at ford resulted in nox sensor costs plus the rear tail pipe cost because the steerer said they could not remove the sensor, plus labor.

alternatively a few folks were able to remove the nox2 sensor them selves and replace it with out difficulty.

my nox2 sensor would not budge and not event heating it with a tourch…has to cut it out. Wasnt fun and I have tons of tools and 50 years of turning wrenches.

 

 

Thanks, so you are saying it looks to you like the downstream (sensor 2) is bad? I will probably have a shop or the dealer do the work. I want to run it a little longer and see if the sensor readings change at all from what I saw now so I will do a little road test this weekend. I am also wondering about the DEF injector. This truck sits a lot, mostly only use it for camping trips.

 

If you take it the shop let them estimate to you what it needs. I wouldn’t prescribe repairs .

 

Was able to get for a longer drive today. At highway speeds I was getting lower nox readings on sensor 2 so I figured that looks good. Dropped the wife of and went and parked. Truck is fully at operating temperature snd idleing in park, nox 2 is now reading 1997, essentially not reading anything right?

 

And now both are not giving actual readings.