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U0629 Lost Communication With A/C Refrigerant Temperature Sensor B

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

A second refrigerant temperature point is typical where an electronic expansion valve meters flow, for instance at the evaporator or chiller outlet, so that superheat can be calculated from temperature and pressure. U0629 is logged when the controller stops getting answers from temperature sensor B over its serial link. A digital sensor can diagnose its own measuring element, so U0629 points to the transmission path or to a sensor that has gone completely silent, not to a merely implausible reading. When U0628 is stored as well, a combined pressure/temperature unit B or its line is the likely common cause. No manufacturer documents U0629 publicly. Often only one operating mode, such as battery chilling or heat pump, is lost. Analog circuit faults are P0EC1 to P0EC4.

Likely causes

LIN line or connector at temperature sensor B open, shorted or corroded
Combined pressure/temperature unit B failed (often together with U0628)
Fuse or supply feed for sensor B open

Symptoms

  • Battery cooling or heat-pump mode unavailable
  • Uneven or reduced A/C performance

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–450
Time0.5–2.5 h

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

Parts involved

ac refrigerant temp sensor bvehicle bus wiring

Related codes

U0627U0628P0EC1P0EC2P0EC3P0EC4

References: en.wikipedia.org, archive.r744.com, law.cornell.edu.