P0B38: Motor Electronics Coolant Pump B Control Circuit/Open
What to do
If charging has quietly become slower than it used to be, mention it — that is a real and frequently missed symptom of this fault, because the car limits the charging rate to protect electronics it can no longer cool. Check the coolant level in the electronics circuit, which on most vehicles is a separate reservoir from the engine's and is easy to overlook. Do not ignore it because the car drives normally: the damage this prevents is to the inverter and charger.
What this code means
The control circuit for the second motor electronics coolant pump is open or faulty. Pump A and the family's reasoning are at P0A05; what a second pump adds is the subject here.
A second pump almost always means a second cooling loop. Power electronics do not all want the same temperature: the traction inverter runs hot and hard in bursts, while the on-board charger and DC/DC converter run warm and steady, often at times the car is not moving at all. So many designs split them into separate circuits with their own pumps, sometimes joined by a valve that can link them when one needs help. Which loop pump B serves therefore decides what overheats when it fails, and that is a model-specific question worth asking before anything is assumed.
The consequence that catches people out is that these pumps run when the car is switched off. The electronics need cooling during charging — which is when the charger works hardest and the vehicle looks completely inert — and often for a period after shutdown, to carry heat out of components that would otherwise soak. So a failed electronics pump frequently shows up as charging that has become slow, because the car protects the hardware by limiting the rate rather than by refusing outright. Nobody connects a slow charge to a coolant pump, and the car does not say so.
Likely causes
Start here
- Faulty inverter coolant pump · moderate
- Corroded or loose connector · easy
Less common
- Open circuit in pump wiring · moderate
- Low inverter coolant level · easy
- Air trapped in the inverter cooling circuit · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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