2019 M2 Competition Overheating Warning, P0128 Code

ClearTheCode admin answer:

1. Understanding the Codes and Symptoms: The P0128 code, "Coolant Thermostat (Coolant Temperature Below Regulating Temperature)," typically indicates that the engine isn't reaching or maintaining its optimal operating temperature. This is often due to a thermostat stuck open. However, your symptom of a "Coolant Temperature Too High" warning is contradictory and suggests a potential sensor issue or a more complex thermostat failure mode.

2. Common Causes for Cooling System Issues on S55 Engine:

   • Faulty Thermostat: The most likely culprit for P0128. An electronic thermostat can fail by getting stuck open (causing P0128) or, less commonly, stuck closed or partially closed, which would lead to overheating. A failing thermostat can also cause erratic temperature readings before complete failure.
   • Faulty Coolant Temperature Sensor: A malfunctioning coolant temperature sensor can send incorrect readings to the ECU, leading to false warnings (both high and low temperature) or incorrect thermostat operation. This would explain the contradictory symptoms.
   • Electric Water Pump Failure: The S55 uses an electric water pump. If it's failing or operating intermittently, it can lead to poor coolant circulation, causing localized hot spots and rapid temperature fluctuations, triggering both high temp warnings and potentially confusing the thermostat's operation.
   • Low Coolant Level: While simple, a low coolant level can cause air pockets, leading to inaccurate temperature readings and poor heat transfer, resulting in overheating warnings.

3. Troubleshooting Steps:

   • Check Coolant Level: Ensure the coolant reservoir is filled to the correct level. A low level can cause the pump to cavitate and lead to erratic temperature readings.
   • Visual Inspection: Look for any coolant leaks around the thermostat housing, water pump, and hoses.
   • Live Data Monitoring: Using an OBD-II scanner with live data capabilities, monitor the coolant temperature sensor's readings in real-time. Compare it to the dash gauge (if available) and observe if the temperature rises steadily and then stabilizes. Erratic jumps or drops could indicate a faulty sensor.
   • Thermostat Operation: If the engine struggles to reach operating temperature or the temperature fluctuates wildly, it points strongly to a thermostat issue.

4. When to See a Shop:

   • Persistent Warnings: If the "Coolant Temperature Too High" warning recurs, stop driving immediately to prevent engine damage.
   • Complex Diagnosis: If you've checked the coolant level and visually inspected for leaks, and the issue persists, professional diagnosis is recommended. Pinpointing whether it's the sensor, thermostat, or water pump requires specific diagnostic procedures and tools.
   • Repair Complexity: Replacing the thermostat or water pump on an S55 engine can be involved, requiring specialized tools and knowledge, including proper bleeding of the cooling system. It's often best left to a qualified BMW technician.