Make sure its not a problem of going from closed loop (cold) to open loop (warm). That is when the oxygen sensor provides signals to the ECU. The test procedure is outlined in a good repair manual or online at sygnus x1. You will need a ohm meter. Check the oxygen sensor for a open heater circuit. You can also check the throttle position sensor. Sometimes they stick and don't come back to idle even though the throttle plate has. You will need a ohmmeter an a set of feeler gauges and a offset (s shaped) Phillips screwdriver. Work with a paper towel under the TPS to catch a screw if dropped.
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/