EPA07/10/GH14 DD Platform Electronics and Troubleshooting Manual, shows SPN3216 as relating to the SCR Inlet Nox Sensor. It does not show the FMI 20, but shows FMI 21. FMI 21 is for Sensor -Drift (Low Box). Just using generic information on Failure Mode Indicator (FMI) 20 is for data drifting high. So my guess would be would be your SCR Inlet Nox Sensor is drifting high. You did not state what engine you have in truck & without last six digits of VIN it is hard to give precise information.
Had same , along with 3226,46,, freightliner replaced turbo, then wire harness then sensors.. the whole time I knew it was the one box but if you got the warranty they want to wait and change that last.. had it changed 2 months ago and not an issue since,, prior to that it was in shop 4 times in 3 months