Network, generic

U029E Lost Communication With NOx Sensor B

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modePossible

Sensor B usually measures NOx at the outlet, behind the SCR catalyst, so the SCR efficiency verdict depends on it. U029E is logged once its CAN messages fail to reach the engine controller. GM monitors the post-catalyst frames the same way as for sensor A and also treats a loss as Type A; Ford's 2023 diesel summary reacts after more than 1 s, and Ford's 2014 code list shows it with MIL. Typical causes are installation errors: Volkswagen's tech tip 01-18-09TT for the 2015 Golf, Jetta, Passat and Beetle 2.0 TDI tells technicians who see U029E after campaign 23Y8 to look for swapped plugs at the front sensor and its adapter and an unseated rear sensor plug. Ford's SSM 50455 ties it on 2021 F-Super Duty 6.7 L to an incomplete PCM reprogramming. The tailpipe check stays open.

Likely causes

Sensor plugs swapped or not fully seated after service work

Water or corrosion in the NOx sensor connectors

Incomplete or wrong engine control module programming

NOx sensor B control unit internally failed

Symptoms

MIL on
SCR efficiency monitor not completed
Engine derate on some heavy trucks

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€0–900

Time0.3–3 h

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

Parts involved

nox sensor bvehicle bus wiring

References: static.nhtsa.gov, static.nhtsa.gov, gsi.ext.gm.com, fordservicecontent.com.