Alright, so I might catch hell for this but I searched and couldn't find an answer.

It's a 1994 22-RE 5-speed 2WD Pickup, completely stock.

I was driving to work the other night and noticed my CEL come on. I did the paper clip thing and got a code 21 which is for the O2 sensor. Which makes sense because it runs a little rough, especially when it's still cold. It doesn't hesitate as much when it warms up. And I've also been getting ˟˟˟˟ty gas mileage so I'm hoping this will fix that because gas is killing me right now.

So my question is if there is a way to determine which O2 sensor needs to be replaced without a scanner, and if it's something I can do myself. I know there's one before the cat and one after since this is a California truck. I'm somewhat mechanical and I love doing things myself so if anybody could help me out I'd appreciate it. My only other option is to just gamble and buy one and hope that it works, worst case I have spend another 100 bucks if I guessed wrong.

Also, will I have to get a new cat? Thanks in advance and any help would be appreciated.

 

21 means a prob with the heater on the 1st O2
28 means a prob with the heater on the 2nd O2

 

I didn't know that. I thought it was just for a O2 sensor in general. So that means the 1st (before cat) sensor needs to be replaced? Is that a difficult fix?

Sorry if these are noob questions but I'm kinda new to Toyotas. I've pretty much only worked on Civics before.

 

yeah pre-cat sensor, it means the ecu sends 12v to the sensor to heat it and doesnt see it return. could be an ecu, sensor or wiring.
sensors is 4 wire- 2 for heater, 2 for signal.

 

Ok, thanks for the info.