U0623 code meaning
Lost Communication With Coolant Pump A
Network system Generic code U06xx codes Check engine light
What U0623 means
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.
U0623 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.3 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0623
- 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
Common causes of U0623
- Most 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
- Less likelyPump electronics failed
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Coolant pump aVehicle bus wiring
What each part of U0623 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 23 | The exact fault in that area. |
What to do next
- Note U0623 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0623
What does U0623 mean?
U0623 means "Lost Communication With Coolant Pump A". Coolant pump A is an electric pump whose speed is set over a digital link, typically LIN, and which reports its status back. It is a generic code, so it means the same on every make.
Is it safe to drive with U0623?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will U0623 fail an emissions test?
Yes, it can. U0623 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U0623 to fix?
Difficulty: medium. A typical repair takes 0.3 to 2.5 hours of work. This is a job for a workshop.
Will U0623 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.