Specifically, the heater for the FRONT O2 sensor.

Unplug the sensor, get an ohm-meter. Measure the resistance between the 2 same-color (black?) wires. They should be 15 to 40 ohms. Then check if either one has continuity to ground. They shouldn't. If it fails any of this, get a new sensor.

If those checks are OK, then you gotta start looking for screwed-up wiring in the car.