While driving my 2000 4-Runner to work this week the Check Engine light came on and with a PO125 code. I've reset the code about 3 times and it continues show up within 5 minutes after I restart the engine. From the excellent articles on this forum it appears that the front sensor is the likely cause. The Runner has a California spec. system with 63K miles.

Is there any other tests or checks I should do before replacing the sensor? Is an aftermarket sensor acceptable or should I stick with the OEM stuff?

Thanks for the great forum.... I found it while doing a search on the code number.

Jeff