Where are the sensors located on the 2003 Buick Rondezvous?

SOURCE: Buick Skylark won't start. intermittant problem

Had similar problem Does the 'theft system' light come on or blink? If so, get it checked out by a good electronic troubleshooter. If it doesn't start, try locking and unlocking driver door twice, and it should reset and start.

SOURCE: 1997 buick lesabre has intermitant speedometer/odometer

Sounds as if it may be an instrument cluster issue. Check the plug at the cluster, if it is ok then travel to your nearest GM dealer and ask them to see if there is a software update addressing your issue. They will likely charge you a diagnostic fee, but it will be cheaper than throwing parts at it in the long run.

SOURCE: Low Coolant light flashing on 1996 Buick Regal 3.8

The low coolant sensor should be located in the passenger side radiator tank, You need to take it out and clean the sludge off of it for it to work right. Or you can just unplug it and forget about it, the light will go out and it won't cause any problems.

SOURCE: Low coolant sensor on a 1998 buick

The coolant sensor should be located near where the upper radiator hose is attached to the engine and has a wire connector on it with 2 wires. It's the only sensor near there. Then there's the sensor that'll be attached to your reservoir tank somewhere along the firewall

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SOURCE: my 1998 buick regal non turbo 3800 radiator

the electric cooling fan for the radiator will not turn on when the engine reaches normal operating temp, its only supposed to come on if the engine temp goes over the normal temp. (around 220 degrees)
it is normal for the fan to come on any time the a/c or defroster is turned on.