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Norskeman's avatar
Norskeman
Explorer
Jul 25, 2014

In the shop - EXHAUST FLUID QUALITY POOR

EXHAUST FLUID QUALITY POOR message
June 12 36619 miles. Cleared after adding 2 ½ gallons of DEF and driving 5 highway miles. Codes read on my Edge Insight CTS:
P20EE SCR NOx Catalyst Efficiency Below Threshold Bank 1
P207F Reductant Quality Performance
P229F NOx Sensor Circuit Range/Performance (Bank 1 Sensor 2)

July 10 37320 miles. Cleared at 37343 miles – same afternoon – highway driving
P20EE SCR NOx Catalyst Efficiency Below Threshold Bank 1
P207F Reductant Quality Performance

Called the dealer and explained my issue and he recommend clearing the codes and bring it in again if it repeats. I discussed the filling of the tank and that there is no more room to add more to the tank. We discussed purchasing DEF and he told me to be cautious of purchasing when it is stored in direct sunlight. I explained where I purchased DEF and that was not the case.

He said if the message comes back bring the truck in and it will be fixed - no appointment required.

July 24 37834 Message came back
P20EE SCR NOx Catalyst Efficiency Below Threshold Bank 1
P207F Reductant Quality Performance

Truck is in the ship for repairs - most probable cause is NOX sensor #2 - they have been replacing a few of them. DEF quality has not normally been the problem.


If #2 sensor readings are "faulty" the ECM is programed to assume it is due to DEF quality.

When using two NOx sensors in a urea SCR system, the upstream sensor is used to estimate what amount of DEF needs to be injected ahead of the SCR catalyst to achieve optimal NOx conversion. The downstream (post-catalyst) sensor is then used to check the results. Under ideal conditions, NOx conversion will be high and very little ammonia slip will occur.

7 Replies

  • Back from the dealer - Tech replaced NOX sensor #2. Ran DPF regen - ran reductant quality test - passed all tests. Cleared codes - rechecked OK.
  • This is an erroneous message as the computer is getting a sensor reading that makes its programming conclude that the DEF is bad. DEF is good for at least 6 months unless you live in an area with 100 plus degree days during that time.

    More likely a bad NOx sensor that is providing bad readings to the computer.

    I buy DEF at the pumps as much as possible as it will be fresher than anything I buy at a auto store in a box. When I do buy 2.5 gallons to have in the garage it is kept in a dark area that is as cool as possible.

    The new Cummins also use DEF as do the current Ford diesel engines. DEF keeps the motor's oil cleanier with less soot buildup which is important with diesel engines. Fleet operators love DEF as it keeps their overall operating costs down.

    The problem with some GM trucks is the first generation NOx sensors which should get better over time. I saw the same problems with the first cars to use catalytic converters and their oxygen sensors that had a relatively short life.

    Difficult with any of these systems is that the mechanics tend to blindly follow the error codes and with a bad sensor it is garbage in and garbage out.
  • GMs are really finicky about exhaust fluids as explained tome by an engineer whose company makes about 85% of the fluid. Told me not to worry about it unless it goes into "limp" mode. I got tired of the lite and traded for a Cummins. Problem solved it seems.
  • wow, nobody can figure out the 10 year old techno in my sub. I throw $500 at CIL lights and suffer peculiar transmission issues. Good luck with your DEF!
  • I understand that there is an expiration date on DEF jugs. I always get mine at bulk pump at truck stops.