What are the symptoms of a faulty BMW electronic thermostat?

[Toby]

There are two symptoms of a faulty BMW thermostat: 1. One is when it fails in the open position, causing slow water temperature rise, preventing the engine from operating under normal conditions. Untimely repairs may lead to engine damage. In winter, slow water temperature rise results in no warm air. 2. The other is when the thermostat fails in the closed state, directly causing overheating and severe engine damage. Here are methods to diagnose thermostat failure: 1. After a cold start, immediately open the radiator filler cap. If the coolant in the radiator remains still, the thermostat is functioning normally; otherwise, it indicates malfunction. This is because below 70°C, the thermostat's expansion cylinder contracts, keeping the main valve closed. Above 80°C, the cylinder expands, gradually opening the main valve to allow radiator coolant circulation. 2. If water flows at the radiator inlet pipe when the temperature gauge reads below 70°C, with warm coolant, it indicates the main valve isn't sealing properly, causing premature major coolant circulation. If the thermostat isn't faulty, check if water flows from the engine outlet pipe shortly after temperature rise - detectable by feeling through the pipe. If not, the issue may be a faulty water pump or blocked engine coolant circulation, which is more complicated to repair. 3. If coolant still flows from the radiator's upper tank inlet during cold engine operation, it means the thermostat's main valve cannot close. When engine coolant temperature exceeds 70°C with no flow at the upper tank inlet, it indicates the main valve cannot open properly, requiring immediate repair.