P0A05
Motor Electronics Coolant Pump Control Circuit/Open
- Severity
- High
- Driving
- Drive only to a garage
The circuit that drives the motor electronics coolant pump is open - a broken wire, a disconnected plug, or a failed pump winding. Hybrids and electric vehicles have a second cooling loop, separate from the engine's, that cools the inverter and the motor electronics. It has its own pump, its own radiator and often its own reservoir - and topping up the engine coolant does nothing for it. This pump is the only thing circulating that loop; there is no belt-driven fallback as there is on the engine side. With it not running, the inverter heats up quickly under load, and the vehicle will cut power hard to protect it.
Possible causes
- CommonFailed coolant pump
- CommonCorroded or damaged connector at the pump
- CommonBlown fuse or failed relay in the pump supply
- CommonWiring fault between the module and the pump
- PossiblePump seized by debris or by coolant that has degraded
- RareFailed driver inside the control module
What you might notice
- Severely reduced power, especially on hills or after sustained driving
- Hybrid or electric drive system warning message
- Overheating warning for the inverter or drive electronics
- Vehicle refusing to charge, or charging much more slowly
- Check engine light on
What to check, in order
- Check the fuse for the pump before anything else. It is the cheapest item in the chain and a blown fuse looks exactly like a failed pump from the module's side.
- Check the 12 volt battery first - it is the most common cause of scattered hybrid system codes.
- Command the pump on with a scan tool if the vehicle supports it, and listen or feel the hose for flow. A pump that runs when commanded moves suspicion to the wiring.
- Check the coolant level in the motor electronics reservoir. Running a pump dry damages it, so a low level may be both the cause and the consequence.
- Measure the pump winding's resistance against specification.
- Do not disturb anything orange.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.