2011 LML bizarre problem...

[SID6FIVE]

Hello all...
I've got a 2011 silverado K2500 at the shop...DIC shows DEF quality bad, truck limited to 55 mph...now for the bizarre part...the tech 2 will read codes but not scan data from the ECM...it will read all the other controllers (scan data and codes)...all powers and grounds are okay...GMLAN checks out fine with no shorts or opens...proper bias resistor readings (120 ohms at each end separately, 60 ohms together)...normal bias voltage readings on each pole (approx. 2-3 volts)...so, my brain says "obviously it's gotta be a bad ECM". New ECM was installed and flashed, and...same problem...I've rechecked the GMLAN section by section and the powers and grounds and even bypassed the Glow Plug Control Module and the TCM with jumper wires and...no change...it's looking like I've got a BAD NEW ECM...but what's the likelihood of that?! Has anyone else run across anything like this? The BAD DEF message and the 55 mph limit won't reset without scan data being available to the other modules...
Thanks in advance,
SID6FIVE

[cabletech]

Do you have another tech2 to try?
From what I hear/read, most poor quality def problems are a bad NOx sensor ( ECM doesn't think def is changing NOx enough). I assume the ECM has all the latest updates?
One other thought.... Is there any evidence that a mouse has chewed on any of the wiring in the truck. It has been cold enough that a mouse may find the engine compartment nice and warm.

Fwiw... I'm sitting at my desk at work just across 35w from Rosedale chev (cty c & fairview)

Jay

[SID6FIVE]

Yep...tried a couple of tech 2's...even updated one to current software (wasn't very far out-of-date)...found nothing physically wrong with the wiring...tech 2 reads all other modules fine, ECM is actually at the far "end of the line" of the GMLAN, TCM is next nearest...been testing thru the TCM connector...no shorts or opens found anywhere in the GMLAN...even jumpered across connectors to eliminate most of the wiring (connected ECM more directly to the dlc)...retained proper bias resistance...still reads codes but not scan data...stuck on this side-track before I can even diagnose the DEF problem...
Sid

[SID6FIVE]

...and the solution: an aftermarket module hidden way up in the dash...connected in series with the data link connector, with its own DLC mounted in place of the factory part...the module failed in some way which created the "DEF QUALITY" message and related codes, latched them in the ECM, and corrupted the scan data from the ECM...once the aftermarket module was disconnected and the ECM was replaced, the truck ran normally and all scan data was readable...