From the Ross Tek wiki:
18613/P2181/008577 - Ross-Tech Wiki
18613/P2181/008577 - Performance Malfunction in Cooling System
Possible Symptoms
Malfunction Indicator Light (MIL) ON
Reduced Heater Output - Cold toes
Incorrect Temperature Gauge Readings
Increased Emissions
Possible Causes
Faulty Coolant Sensor
Faulty Coolant Thermostat
Faulty Wiring, Connections
Note: This fault code sets when engine coolant temperature mapping is lower and out of expected range based on operating conditions of the engine.
For example: Engine warms up, thermostat opens and stays open, coolant temperature drops below normal operating range. The ECM knows the coolant temperature should stay within 80.0...110.0 °C typically after warm up. Thus a drop in coolant temperature below normal operating temperature is not possible without a Malfunction in Cooling System.
Possible Solutions
Check G62 - Engine Coolant Temperature Sender
Inspect wiring for G62 (Coolant Migration, damage, corrosion, etc)
Check (if applicable) G83 - Radiator Outlet, Coolant Temperature Sender
Inspect wiring for G83 (Coolant Migration, damage, corrosion, etc)
Check Coolant Thermostat
Use Graph function in VCDS, Measuring Value Blocks - MVB
Start Engine cold, graph Coolant Temperature Sender (G62) Typically MVB Group-001
Hold RPM around 1,500rpm and watch coolant temp increase in Graph until thermostat opens, indicated by a slight drop in coolant temp
If coolant temp drops below minimum operating temperature (80°C) or never reaches operating temperature, (verified by external thermometer) suspect failed thermostat
Vehicles with both G62-Engine Outlet, Coolant Temperature Sensor and G83-Radiator Outlet, Coolant Temperature Sensor
Use Measuring Value Block group MVB-130 instead on MVB-001
G62 should increase in Temperature before G83 increase when engine coolant is cold
G83 will increase when the thermostat opens and after G62 reaches operating temperature (80°C)
If G62 and G83 both increase while engine warms up, suspect failed thermostat