U0623Lost Communication With Coolant Pump A
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- 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.
- What you may notice
- 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
- Likely causes
- LikelyChafed or pinched LIN harness shared with other actuators (GM 23-NA-102)
- PossibleOutdated hybrid or inverter controller software (Stellantis 08-399-25)
- PossibleBlown fuse, relay or open supply/ground to the pump
- RarePump electronics failed
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €50 – €800
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.