Low coolant warning
I have a 2004 Vauxhall Zafira 2.0 and have just replaced the coolant expansion bottle, swapping the low coolant sensor in it with an original one. Now I have a message on the dashboard saying the coolant level is low, even though it is topped up. M Badcock
From your letter, I am assuming that this problem has only occurred since the coolant expansion tank was replaced. If this is the case, a sticking float in the new expansion tank or a poor connection to the new sensor may be the most likely cause.
The low coolant sensor works in conjunction with a float within the expansion bottle. I would first check that the float is free to move within the expansion tank and that it is sitting correctly above the sensor. If there is no fault with the float or sensor, I would inspect the condition of the wiring to the sensor, to ensure that the problem is not being caused by a shorting or broken wire.