I have got a 91 Millenia 2.5L
It gives me two codes, one is P0510 about throttle position sensor
the other is about downstream O2 sensor.
The problem is that the car has a very low engine rotation speed when I don't press the accelerator pedal, around 300 rpm, and the car will even go off. Is the problem caused by the TPS or the O2 sensor? Or some other possibilities?

 

I have never seen a bad TPS on a millenia but i suppose it's possible. It is possible the throttlebody has been tampered with would be my first thought on that code. A misadjusted TPS could cause what you describe.A downstream O2 monitors the efficiency of the converter it is behind. Your car has 5 O2 sensors total only 2 of which are used for actual adjustments. The others monitor cat efficiency only. You will need the exact code in order to diagnose the O2/converter problem. It sounds like it needs a good going over. Vac lines/connectors/plugs/wires/throttlebody and injector flush.

 

thanks for the reply.
The other code is: P0420 Catalyst system efficiency below
Can this code also cause the low rotation speed problem?
the garage suggest me replacing the downstream sensor, which they asked 350 (including labor) each, i think it's crazy.
I just want to fix the problem of low rotation speed now. My car has only about 15 to 16 MPG now.
Maybe I'll change the TPS.
And if i don't replace the downstream sensor, what bad will this do to my car?

 

by the way, if the check engine light is still on after i change the TPS, how can I turn it off?