3B99
Moderate3B99 — Coolant Thermostat and Electric Pump Fault Confirmed
Description
What 3B99 Means
3B99 means extended diagnostics confirmed a fault in the map-controlled thermostat and the electric coolant pump. The code falls under the cooling category and carries moderate severity.
The BMW cooling system with an electric pump and map-controlled thermostat runs under DME control. The electric pump circulates coolant regardless of engine speed, and the map-controlled thermostat regulates flow through the radiator to hold the target temperature. When either component fails, the thermal regime breaks down and the engine can overheat.
Symptoms
Symptoms
- Owner complaints of engine overheating.
- Overheat messages may appear on the instrument cluster.
- Coolant temperature can exceed the allowable range.
Causes and fixes
Diagnostics and Repair
Component Replacement
- Replace the electric coolant pump.
- Replace the thermostat.
After Replacement
- Clear the stored fault codes from the DME.
- Fill the cooling system with coolant.
- Bleed the system with the service function to remove air.
- Fully warm up the engine.
- Let it cool down completely.
- Set the coolant level.
- Recheck that the cooling system works correctly.