This is a quote from the w/s manual.
Diag code 21,28: MAIN OXYGEN SENSOR CIRCUIT
The main oxygen sensor senses whether the air fuel ratio is richer or
leaner than the stoichiometric air-fuel ratio.
Aftr more technical info on its constuction, the crux is that the sensor generates a voltage
signal sent to the ECU which uses this signal to increase or reduce the injection volume
to keep the air fuel ratio at an even value near the stoichiometric air fuel ratio.
The main oxygen sensors include a heater which heats the element.this heater is
controlled by the ECU.
code 21 is left bank, 28 is right bank.
Don't know if this is relevant.......there is also the cold start injector is it turning off?
and the fuel line pressure...
Stick at it, you will be an expert and we can all call on you!