NHTSA ID Number: 11034394
Manufacturer Communication Number: 43-088
TSB/Document Date: 2026-07-02
Summary
"An optional software update will be available on 07/06/2026 for chassis equipped with EMY 2021B-2023PACCAR MX-11 and MX-13 engines. This update provides customers with additional time to service theirtrucks before a DEF-related derate occurs. The software is being offered to customers in alignment withrecently updated regulatory agency guidance."
6/29/26, 1:44 PM
Vehicle Service Bulletin
43-088: PACCAR
MX Engine Model Year 2021B-2023 DEF
Derate Time Extension
6/29/2026
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43-088
Subject
PACCAR MX Engine Model Year (EMY) 2021B-2023 Diesel Exhaust Fluid (DEF) Derate Time Extension
Whats New Abstract
An optional software update will be available on 07/06/2026 for chassis equipped with EMY 2021B-2023
PACCAR MX-11 and MX-13 engines. This update provides customers with additional time to service their
trucks before a DEF-related derate occurs. The software is being offered to customers in alignment with
recently updated regulatory agency guidance.
Condition
This regulatory agency-aligned software update provides vehicle owners additional time to service their
trucks before a DEF-related derate occurs.
Low or empty DEF tank levels trigger a fluid-level-based derate rather than a time-based limit. Previously,
vehicles would progressively derate to 5 mph as the tank emptied. Now vehicles will progressively derate
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6/29/26, 1:44 PM
Vehicle Service Bulletin
to 25 mph.
Poor DEF quality and DEF-related system faults trigger a time-based derate. Previously, operators had 4
engine hours before the vehicle was limited to 5 mph. Now operators have 160 engine hours before the
vehicle is limited to 25 mph.
All vehicles require Diesel Exhaust Fluid to operate properly.
NOTE
he following items clarify what to
T
expect after the software update has
been completed.
Turning the key off and back on
will not reset the derate timers
once a DEF error has occurred.
Inducement time counters will
not increase while the key is
off.
Chassis Affected
All chassis equipped with an EMY 2021B-2023 PACCAR MX-11 or MX-13 engine
Action
Informational.
An optional software update is now available to allow more time to service a chassis before a DEF
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Vehicle Service Bulletin
related derate occurs.
Warranty
This bulletin is for informational purposes only and is not a proactive campaign.
This optional software update is customer pay and is not covered under warranty.
NOTE
This software update does not
eliminate the requirement to use DEF.
Any warranty claim that is the result
of a failure to use DEF as required or
misuse/tampering with the
aftertreatment system may be denied.
Procedure
NOTE
Software will be available on
07/06/2026. Only perform this
software update 07/06/2026 and later.
NOTE
This software update does not
eliminate the requirement to use DEF.
Any warranty claim that is the result
of a failure to use DEF as required or
misuse/tampering with the
aftertreatment system may be denied.
Please see the procedure in the Attachments section below for instructions on updating the software.
https://paccarnet.sharepoint.com/sitepages/vehsvcbltin.aspx?ctx=eyJ3ZWJVcmwiOiJodHRwczovL3BhY2Nhcm5ldC5zaGFyZXBvaW50LmNvbS9zaX…
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Vehicle Service Bulletin
Attachments
43-088: PACCAR MX EMY 2021B-2023 DEF Derate Time Extension - Procedure
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TSB/Document ID: 43-088
Replacement Service Bulletin Number:
MFR Communication Date: 2026-06-29
MFR Internal Campaign ID/Software Version:
Communication Type: Service Bulletin/Repair Instructions
NHTSA Components: ENGINE
MFR Component System:
MFR Component Subsystem:
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