Why does the service engine, trac off and abs lights come on when I hit a bump, on a 2004 pontiac grand am. I found out I needed front brakes (inside) so replaced the front brakes (rotors were fine). The lights used to go off after I turned off the car and back on again, but now they stay on all the time, even after my new brake job.

There maybe a short in the ground wires somewhere.or one of the sensors for the brakes needs to be check.

Http://www.ricksfreeautorepairadvice.com/repair-topics/brakes-repair-topics/abs-and-trac-off-lights-on-gm-vehicles

Shops are reporting huge failure rates on ABS sensors in some late model GM vehicles where the ABS sensor is integrated with the wheel bearing/hub. These sensors are in two styles. One style bolts the sensor directly on the wheel bearing. The sensor counts the notches on the ABS tone ring inside the wheel bearing. The clearance between the sensor and the tone ring is critical. The problem occurs when rust builds up between the sensor and the wheel bearing. This phenomenon is called "rust jacking," where the rust literally pushes the sensor upwards, increasing the gap between the sensor and the tone ring to the point where it reads intermittently. If you have this style sensor, remove it and lightly sand the rust (lightly being the key word). Then apply a light coat of marine grease on the bare metal and re-assemble. You may want to try applying a thin bead of RTV silicone around the outer edge to prevent water from getting under the sensor.
The second and more common style has the ABS sensor built into the bearing/hub. This unit CANNOT be serviced. You must replace the entire bearing/hub.