Detailed repair instructions at the Pad and Wheels site.
Also see Oxygen Sensor Analysis.
The sensor helps the ECU control the air-fuel ratio. The oxygen sensor is in the exhaust elbow between the turbo exhaust outlet and the primary catalytic converter. Routine O2 sensor maintenance is replacement at 50,000 miles.
A bad O2 sensor will not necessarily trigger a Check Engine Light. The ECU only triggers the CEL and stores a code if the O2 sensor returns a value that is out of ECU spec. If it reads a little leaner or richer than it should, the ECU won't pick up the defect untill the readings don't make sense to the ECU.
If you pull the O2 sensor out, it should have what looks like normal black carbon deposits on it. If it has alot of white chalky looking powder/residue on it, it is probably bad.
The sensor is not a screw-in style that is used by most other cars, but actually has a flange. The exhaust component (downpipe or manifold) will have two studs. The sensor with flange and gasket is placed on the studs and then held in place by two nuts. If no sensor is present, there will probably be a block-off plate used.
Popular makes are Toyota, Denso and NGK. Bosch sensors do fit, but measure differently to stock -- there appears to be little functional difference. Regarding generic sensors, you can use one, but make sure you get a generic sensor that is appropriate for the vehicle. Not all 1-wire sensors are the same even if they fundamentally operate similarly and look the same. The internal construction may be different (they are essentially built for different heat ranges and exhaust streams). All reputable manufacturers will give you a recommendation on which generic to use.
85-87 cars use single wire sensors that are located in the manifold. Use a single wire sensor. Located at the bottom of the exhaust downpipe before it bends pointing to the rear of the car.
- Toyota #: 89465-19136 (which superceeded 89465-19135)
- Bosch #: 12051
88 and 89 (both NA and SC) use 4 wire heated sensor located in the downpipe. (The heater was required because the sensor was moved from the bottom of the manifold to after the B-pipe flex section, back by the axle.)
- 2 black wires - sensor heater; white - O2 sensor output; blue - ground
- OR 2 whites - sensor heater; black - O2 sensor output; gray - ground
- Toyota # (NA): 89465-19206
- Toyota # (SC): 89465-19245
- Bosch #: 15711
- Denso #: 234-4051 (SparkPlugs.com)
- NGK #: 2419
Source: http://www.mr2oc.com/showthread.php?t=106726, http://www.mr2board.com/forums/mk-2-mr2-sw20/2828-there-any-way-tell-if-my-o2-sensor-bad.html, http://www.mr2oc.com/showthread.php?p=1588132, http://www.mr2oc.com/showpost.php?p=4517828