Low coolant light coming on - 1998 Chevrolet Venture Cargo

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The CHECK COOLANT indicator illuminates when the coolant level is low in the coolant recovery reservoir. The low coolant level switch is probably in or on the coolant reservoir !

not the pump but air in the system or a broken wire to the coolant level sensor
that sensor at the top of the radiator tank has to be completely immersed in coolant or the circuit is broken and the light comes on
same effect id the wire to that sensor is broken in the insulation

These problems may or may not be related. The Check coolant light means that the coolant level in your radiator is low. If you have checked it and it is not low, then the sensor is most likely defective. The low coolant sensor will not turn the Check Engine light on - even if the coolant really is low. There are many things that can cause the Check Engine light to come on. You will have to scan your Engine Computer to find out why it has been turning the light on.

I did have a problem on one of these cars one time where the Check Engine light was coming on and there were several codes for the Throttle Position Sensor(TPS) and the Manifold Absolute Pressure (MAP) sensor and the car ran really bad. The Check Coolant light was also on. After checking it out I found that the coolant level sensor was shorting out the 5 volt reference circuit (VREF) from the computer and causing the whole problem. Replacing the coolant level sensor fixed the car.

Low coolant doesnt have anything to do with the thermostat. If the coolant bottle was over filled, it will push out, it needs some air space. only fill it to the full line. If the low coolant light is on, but its not really low, the sensor may be bad. Be sure to use the correct coolant DEXCOOL.

If the low coolant light is coming on, there could be a leak in the cooling system and the system may be very low. Sometimes if the cooling system is full and there are no leaks, the low coolant sensor might have a malfunction and turns on the low coolant light.

I think your buick has a pressure coolant system so you may have air in the system and need too bleed it and for the light coming on while on the highway most likely low coolant when your driving the higher rpm the more coolant is drawn into the motor and triggering the light from coolant reservoir running low on coolant

the low coolant light sensor is defective replace the sensor, it is part of the coolant recovery tank

change sensor in water tank

Check you coolant resevoir. The low coolant switch is located in the resevoir so if it is low on coolant there, the light will come on. If it is full, it could be that the switch is stuck due to gunk build up in the resevoir. Hope this helps you out