03/29/2026
DEF System Guidance changes
Lots of misinformation flying around the internet regarding DEF systems since the EPA guidance release on 3/26/26. Forget the internet articles and AI summaries, here are the facts.
Document: EPA IACD-2026-05
KEY POINTS:
1. The EPA does not dictate the technology for emissions, only the air quality parameters.
2. This guidance is addressed to the engine manufacturers as a policy change giving the OPTION, not directing any change, nor setting any timelines for changes.
SUMMARY:
1. Engine manufacturers using approved NOx sensor systems (currently only Cummins, 2007.5-Current) are now allowed to develop and implement (with EPA approval) ECM software updates to use the conversion efficiency data from the NOx sensors to determine if the DEF is of sufficient quality to properly scrub NOx from the exhaust stream VERSUS relying solely on the data from the in-tank quality sensor.
2. This provides another parameter to check DEF quality before activating “Severe SCR Operator Inducement” more commonly known as torque derate or “limp mode” due to incorrect DEF quality data from the in-tank quality sensor.
3. If the conversion efficiency data shows that the DEF is NOT of sufficient quality (downstream NOx levels are too high), the ECM will derate the coach per current programming. The IACD does not provide for changing this action.
KEY NOTES:
1. No part of IACD-2026-05 eliminates the need for DEF level and temperature data, which share the same digital processing component inside the DEF head digital sensor package. Failure of the digital processing componentry (the most common method of DEF head failure) will still lead to derate, due to no temperature or level information, even with modified derate schedules (authorized by IACD-2025-10 for ‘26 and older) which have not yet been implemented.
2. No part of IACD-2026-05 authorizes bypass, impairment, defeat, or disabling the control of emissions of any regulated pollutant. “Deletes” are still NOT authorized and the consequences of doing so are unchanged.
CONCLUSION:
“Classic” DEF head failure will still lead to derate even with the new 2027 and newer guidance (CFR section 1036.111), since the quality sensor is not the only component affected.
A Voyager RV DSS is now, and will remain, a critical tool to mitigate the limitations that come from a failed digital DEF head sensor package.
