why is my "Check Engine -ABS LIGHT -BATTERY LIGHT" ON? on 1997 Buick Riviera

I got me a 97 Buick Riviera Supercharged 3800 series II engine. when i got the car those lights were already on. the vehicle also had a bad pulley on the alternator so i got a new alternator itself and got me some new spark pluggs while i was there . everything works fine car rides smooth but my "abs light, check engine ,and battery light" are still on now keep in mind the battery light appeared after i installed the new alternator but the alernator has been tested various times and passed every test. if u could help it would be great ! thanks.

Asked by for the 1997 Buick Riviera
you have a power issue with the alternaor. the new one may be defective or the wiring may be failing. have an electrical shop re check it for you.

the check engine light is a fuel control issue. you need the code pulled, properly diaged and a bid for repairs.

the abs has a failure and that system has to be checked for the failure as well.