Is that for the MK1 GS300?
DTC 21, 28 Main Heated Oxygen Sensor Circuit
CIRCUIT DESCRIPTION
To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a three way cata-
lytic converter is used, but for the most efficient use of the three way catalytic converter, the air–fuel ration
must be precisely controlled so that it is always close to the stoichometric air–fuel ratio.
The oxygen sensor has the characteristic whereby its output voltage changes suddenly in the vicinity of
the stoichometric air–fuel ratio. This characteristic is used to detect the oxygen concentration in the exhaust
gas and provide feedback to the computer for control of the air–fuel ratio.
When the air–fuel ratio becomes LEAN, the oxygen concentration in the exhaust increases and the oxygen
sensor informs the ECM of the LEAN condition (small electromotive force: 0 V).
When the air–fuel ratio is richer than the stoichiometric air–fuel ratio the oxygen concentration in the ex-
haust gas is reduced and the oxygen sensor informs the ECM of the RICH condition (large electromotive
force: 1 V).
The ECM judges by the electromotive force from the oxygen sensor whether the air–fuel ratio is RICH or
LEAN and controls the injection time accordingly. However, if malfunction of the oxygen sensor causes an
output of abnormal electromotive force, the ECM is unable to perform accurate air–fuel ratio control.
(The main heated oxygen sensors include a heater which heats the Zirconia element. The heater is con-
trolled by the ECM. When the intake air volume is low (the temperature of the exhaust gas is low) current
flows to the heater to heat the sensor for accurate oxygen concentration detection.)
From the manual for the MK1.
Looks like its a narrow band