Electric cooling fans always on

2001 Bonneville SE -I can rebuild an engine mechanically, but when it comes to auto electronics, I'm stumped!!
Over the last several weeks, both cooling fans turn on when I start the car (cold engine). They stay on all the time. My temp. guage works fine and shows the temp. to only get to about 170. The outside air temp. has been between -15 to +5 degrees celsius, so with both fans on, I feel the guage is working OK.
I was told that without the "service engine" light on, reader/scanners will not show any stored codes. Is this right? One suggestion was that a stored code was causing the fans to stay on. But how would you turn it off?
Does this engine have more than one temp/guage sensor? If I remove the 2 pin connector from the sensor under the thermostat, the fans stay running and my guage still reads 170. I expected my guage would cease at the least.
There are three mini relays in the fuse panel for the fans - primary, secondary and series/parallel. Could these be the issue?
Any advice or insight is appreciated. Thanks

Are They stuck on high speed (!2 volts to each motor) or low speed (6 volts to each motor)?

-- if low speed, your low speed relay may be stuck on- pull all three loose and see if they click when you slowly plug them back in....

Also there is a note, that your coolant fan relays are mislabeled underhood on your year car....as noted below

Important: The cooling fan 1 relay and cooling fan 2 relay are improperly identified in the underhood fuse block. Refer to Electrical Center Identification Views in Wiring Systems for proper identification

Ok - Removing all three relays shuts off the fans. If all three are plugged in, removing the Series/Parallel relay shuts off one fan only.

If I disconnect the fans, they each show full 12 volt power, actually 15 volts.

Now, if I disconnect the temp. sensor it shows 4.5 volts. Then STRANGE THINGS happen. If I leave it unconnected for a while, the guage stops registering. But if I keep the multimeter hooked up, the engine picks up to 1100 rpm and the guage reads 200. If I plug the sensor in again, the engine idles down and the guage goes back to 170. The fans remain on throughout.

After repeating this several times, the "Service Engine Sonn" light has appeared.

I assume the a/c is not requested? Fans on H cars are on with a/c request- regardless of engine temp....

Seems I got lucky. My checking the temp. sensor set off the "service engine soon' light. So I rushed over to the auto parts store and put a code reader on the car. CODE P0118 showed up - Engine Coolant Temperature sensor. Needless to say I bought a new one.

After deleting the code I drove home and surprise - the fans were not on!

So I will be draining the coolant to replace the sensor and hope that my problem is solved.

I can't help but wonder if that code was stored in the computer.