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U0623 Lost Communication With Coolant Pump A

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

Coolant pump A is an electric pump whose speed is set over a digital link, typically LIN, and which reports its status back. GM's 2025 tables (25OBDG03A, 25OBDG04B) call it the auxiliary electric water pump and fail U0623 as Type B when its response frame is absent for 2.5 s although the LIN channel is ready and either the actuator relay or the powertrain relay is energised. On the Duramax 3.0 diesel GM bulletin 23-NA-102 names it the heater coolant pump; it shares ECM LIN bus 1 with grille shutters and fans, so chafed wiring sets several codes together. Stellantis bulletin 08-399-25 for the 2024 Dodge Hornet PHEV (also Alfa Romeo Tonale) lists U0623 alongside P2601 and fixes it by reflashing the hybrid control processors. Effects range from weak heating to blocked cabin preconditioning.

Likely causes

Chafed or pinched LIN harness shared with other actuators (GM 23-NA-102)
Outdated hybrid or inverter controller software (Stellantis 08-399-25)
Blown fuse, relay or open supply/ground to the pump
Pump electronics failed

Symptoms

  • Check engine light
  • Reduced cabin heat or failed scheduled preconditioning on plug-in hybrids
  • Other LIN device codes (grille shutters, fans) stored at the same time

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–800
Time0.3–2.5 h

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

Parts involved

coolant pump avehicle bus wiring

Related codes

P2600P2601P2602P2603U0624U062FU0632U0633

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