1993 volvo 940 turbo wagon bulb monitor light up

Make sure all bulbs match in wattage, manufacturer and condition.
Also ensure the connections in the bulb circuit harness connector are clean.
Those bulb monitor sensors are a special kind of relay. It can detect one bulb out. If two bulbs fail, you wouldn't get a warning. Sometimes I find dirty connections at the base of the sensor itself and cleaning those fixes it. The entire affair is installed into a red cylindrical shaped canister in the electrical center. These sensors are very sensitive to resistance differentials. I've gone through this many times with these sensors and the only real way to fix the issue (provided the sensor isn't faulty) is to change every bulb including the third brake light all at the same time, clean every ground and connection point, reinstall the ground tabs back to the body and meter each component out with a decent high-impedance ohmmegger if needed. It's a pain, and sometimes I end up just removing the indicator from the dash cluster and simply inspect each week. They are known to be overly sensitive especially when aftermarket radios and trailer wiring are installed. Note on trailer wiring: always install that into a separate branch circuit, preferably bypassing the unit as it could become damaged and then your headlights will stop working as well. Good luck and keep me posted should you need more help.

check the fuse .and all of the bulbs are working.if ok then change the brake switch.

check the stop light switch right under the brake pedal.

the BOO switch, (Brake On Off) located on or near the brake pedal

hi fisrt check youer brake light blubes
if ok check the brake switch

my bulb light stays on i have replaced all the bulbs and fuses have replaced the brake switch now my top light come on when you switch the fog light switch on what else can be the problem

Sometimes the fuse looks good,but replace it anyway,that sensor in under your dash,its red and round.

brake fluid low, or brake pads need replacing.

make sure bulb 1157 is in socket correctly- offset pins in proper slots of socket, and socket properly installed in taillight assembly- two small tabs one larger tab in proper slots- hint ground terminal is up

behind fuse panel big red round relay and you can normally solder bad connection inside if you look close