Engine light and warning light on.

[pwpanas]

Originally Posted by Chomper

...the code seems to be 21. That means its the oxygen sensor? should I just replace it?...

According to the documention, the problem is either:

Open or short in heater circuit of main heated oxygen sensor (Fr) for 0.5 sec. or more.

Possible trouble areas are:

• Open or short in heater circuit of main heated oxygen sensor.
• Main heated oxygen sensor heater
• ECM

....OR....

Main heated oxygen sensor signal voltage is reduced to between 0.35V and 0.70V for 60 sec. under condition
a) ~ (d). (2 trip detection logic)

a). Engine coolant temp.: Between 80° C (176°F) and 95°C (203°F)
b). Engine speed: 1,500 rpm or more
c). Load driving (EX. A/T in overdrive (5th for M/T), A/C ON, Flat road, 50 mph <80 km/h>)
d). Main heated oxygen sensor signal voltage: Alternating above and below 0.45 V.

Possible trouble areas are:

• Main heated oxygen sensor circuit
• Main heated oxygen sensor

Simply replacing the O2 sensor might or might not fix the problem(s). In theory, the problem could also be in the wiring or the ecu itself. In other words, it's hit and miss if you start swapping parts without going through the proper troubleshooting procedures. In order to do this troubleshooting properly, you'd need a copy of the TSRMs (Toyota Supra Repair Manuals), and you'd need to follow ALL of the troubleshooting/diagnostic steps for each of the possible trouble areas listed above.

For what it's worth, if you're going to be working on your own Supra, I have to strongly recommend that you pick up your own copy of the TSRMs. They will literally pay for themselves several times over, which makes them an excellent investment.

One last observation: if your Supra has more than 100K miles on it, and especially if it has more than 150K miles on it, and your primary/main O2 sensor is the one that came with the car when it was new, I'd suggest you replace your primary O2 sensor regardless...

Originally Posted by Chomper

...Do you know if it can cause the low idling problem...

Yes, it could be responsible for low idling, due to the likely-inaccurate O2 sensor readings being passed back to the ecu in closed-loop.