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U029D Lost Communication With NOx Sensor A

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modePossible

NOx sensor A is normally the engine-out or SCR inlet sensor; its control unit puts NOx and oxygen values on CAN. U029D means these frames have stopped. GM treats it as Type A: its 2016 diesel tables wait 10 s, the 2025 tables (25OBDG04A/B) about 1 s for most frames, at 11 to 18 V with the bus otherwise healthy; on the 2014 6.6 L LML the code also counted checksum errors. Ford's 2023 diesel summary allows just over 1 s. Ford found water in the sensor module plug of 6.7 L F-Super Duty trucks from 2017-2019 and replaced module and harness 14406 (TSB 18-2365). Volvo and Mack name fuses, supply, J1939 wiring and terminating resistors, and on 2023 trucks a start-up timing bug. Deutz uses U029D for the upstream SCR sensor. MIL, open SCR checks and derate are possible.

Likely causes

Water or corrosion in the NOx sensor module connector or harness

Blown fuse, open supply or ground of the sensor module

Sensor CAN wiring damaged or terminating resistor faulty

Engine control software with start-up timing fault

NOx sensor control unit internally failed

Symptoms

MIL on
SCR or NOx monitor not completed
Engine derate on some trucks

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€50–900

Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

nox sensor avehicle bus wiring

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com, fordservicecontent.com.