The computer runs two different fuel programs. Open loop and closed loop. Open loop ignores the o2 sensors and controls fuel based on a default program. Open loop generally supplies much more fuel than closed loop operation. The vehicle runs in open loop until it reaches a preset temperature (usually the 160 degree range). After the engine warms up properly it switches to closed loop. Closed loop operation monitors the o2 sensors and adjusts the fuel delivery to provide maximum efficiency. If the engine doesn't reach the proper temperature because of a faulty thermostat, or if it thinks it isn't reaching the proper temperature due to a faulty sensor... the engine will remain in open loop operation.