Troubleshooting Low-Temp Coolant Pump Activation on a 2015 S65 AMG

A recent inquiry from a local Mercedes owner highlights a common frustration when using aftermarket diagnostic tools for complex cooling system repairs. The customer, working on a 2015 S65 AMG with the W222 chassis, is trying to purge air from the intercoolers after a coolant swap.

Using a LAUNCH X-431 PRO3S+ VERSION 7 scanner, he found the bi-directional option to activate the low-temp coolant pump, but pressing F2 does nothing, even with the battery on and engine off as recommended. The question is whether the scanner is the problem or the car's software is blocking the command.

From our experience at Boca Auto Care, the issue here is likely not a limitation of the LAUNCH scanner itself, but rather a safety logic built into the S65's engine control unit. Mercedes' STAR diagnostic system has specific preconditions for the low-temp coolant pump activation, including a minimum coolant temperature threshold and proper system pressure.

The pump will not run if the engine is cold, even with the key on, because the ECU assumes the pump is unnecessary for air purge at low temperatures. This is a common oversight in aftermarket scanners that do not fully replicate STAR's conditional checks.

That said, the LAUNCH X-431 is a capable tool, and there is a workaround. Some technicians have had success by warming the engine slightly, just enough to bring coolant above 50°C, then retrying the activation with the engine off. Alternatively, the YOUCANIC scanner or a dedicated STAR system may offer more direct control.

If you are stuck, bring the car to us. We can use our factory-level equipment to cycle the pump properly, ensuring all air is expelled from the intercoolers without risking overheating or pump damage.