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U0651 Lost Communication With EGR Sensor A

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modeNo

EGR sensors A to D are generic slots: a maker may assign them to valve position, pressure or another EGR measurement. In GM's 2025 diesel table 25OBDG04B, which has both a high-pressure and a low-pressure EGR loop, sensor A is the SENT position sensor of the high-pressure valve. U0651 is set there if at least 480 out of 600 tests, one every 6.25 ms, find stale data with the line stuck low or high, a message error, or counters that stop, provided the relay supplies more than 11 V and no start is in progress; Type A, MIL in the first trip. A raw signal below 1 % of 5 V on the same sensor is P0405 instead. Other analog faults run through P0405 to P0409 and P046C. EGR flow monitoring stops while the position is missing.

Likely causes

SENT line to the EGR valve damaged by heat or chafing

EGR valve connector corroded or loose

Position sensor in the EGR valve failed

Engine controller input fault

Symptoms

MIL on from the first trip
EGR switched off, possibly louder combustion
EGR monitor not ready

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€60–900

Time0.5–3 h

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

Parts involved

egr sensor avehicle bus wiring

References: gsi.ext.gm.com, law.cornell.edu, en.wikipedia.org.