(1) Fault symptoms The chassis model of this car is E46, equipped with an M43 engine. The fault 

symptoms are as follows: According to the owner, after starting the engine, the coolant level warning 

light (yellow water tank-shaped symbol warning light) on the instrument panel keeps lighting up. 

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(2) Diagnosis and maintenance: Start the engine and conduct actual inspection. No yellow water tank 

symbol warning light is found to be on. Turn off the ignition switch, and after the temperature drops, 

open the coolant reservoir cap. You can see that the liquid level is indeed low. Check the appearance of 

the engine and find no obvious signs of leakage. 

       pressure test was conducted at the mouth of the liquid storage tank and no trace of leakage was found.

  No leakage when the engine is cold does not mean there is no leakage after the engine is running.

  There may also be internal leakage issues. Remove the spark plug and check the internal condition of 

the cylinder. The top of each piston is dry. This eliminates the possibility of the cylinder gasket being 

torn. Connect the diagnostic tool for self-diagnosis. There are no fault codes in the engine system and 

instrument panel. After consulting relevant information, I learned that the coolant level warning signal 

on the instrument panel is transmitted by the coolant level switch, which is located at the bottom of the 

radiator reservoir. The circuit diagram is shown in Figure 1-26. Unplugged the wiring harness plug of 

the coolant level switch and tested the car. Sure enough, a yellow warning light with a water tank 

symbol came on. Plug in the wiring harness plug and the warning light will go out. Add an appropriate 

amount of coolant to the reservoir tank and perform a road test. It is found that a small amount of 

coolant overflows from the edge of the reservoir cap, indicating that the reservoir cap is poorly sealed.

  Replace the reservoir cap and the fault is completely eliminated. 

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(3) Summary When the coolant is sufficient, the coolant level switch is closed; when the coolant is 

missing, the coolant level switch is opened and a warning message appears on the instrument panel.

  Depending on the model configuration, the coolant warning information is displayed in a different 

manner. A The formula is also different. The dashboard of some models displays text information 

through an LCD screen, and the dashboard of some models provides alarms through relevant warning 

lights. For this type of fault, not only the coolant level but also the coolant concentration must be 

checked, because the concentration will affect the float height of the coolant level switch, which in turn 

affects whether the switch can be closed.