Could be the O2 sensor. This test is from the BB faqs:

Oxygen Sensor Simple Diagnostics. [Herb Goltz] You don't need to remove the sensor or jack the car up to check it. You need a digital voltmeter with an impedance of at least 10 megohms to prevent damage to the ECU. First, trace the leads from the sensor up to the firewall. For your model there will be 3 wires, two of which go into a connector. They are just wiring for a heater to get your sensor up to temp quicker. The wire you are interested in is a single shielded wire-- depending on the model, it will have various colors. There will be a spade connector underneath a bullet shaped black cover. Just pull the cover back. Warm the car up to operating temperature. Uncover the spade connector-- leave all the wires plugged in and the car running. Set your multimeter to DC volts, and the range to <2V. Ground the negative probe of the multimeter to chassis ground (like the strut tower) and the positive to the connection at the O2 sensor. You should see voltages oscillating regularly from around 0.1-0.9V. If voltage is steady or zero then you likely have a bad O2 sensor.
Oxygen Sensor Problem Diagnostics.
a. Bosch Diagnostics: Test for a rich mixture as follows:
Disconnect the sensor lead to the control unit.
Run the engine at 2500 rpm
Artificially enrich the fuel mixture on electronic fuel injected engines by removing and plugging the vacuum line to the fuel pressure regulator.
If the voltmeter rapidly reads .9 volts, then the oxygen sensor is correctly sensing a rich mixture. But, if the voltmeter responds sluggishly, or if it stays below .9 volts, try running it at 3,000 rpm for a few minutes, then check again. No improvement means you buy a new sensor.
Test for a lean mixture as follows:
Induce a small vacuum leak
If the voltmeter rapidly drops to .2 volts or below in less than a second, then the oxygen sensor is correctly measuring the lean mixture. But if the voltmeter responds sluggishly, of if it stays above .2 volts, give it the 3,000 rpm treatment and try again.
If no improvement, then the sensor should be replaced.