Bad O2 sensor?

I have a 97 voyager 2.4 motor. I am getting bad gas milage. The check engine light is giving me code 21. I want to fix it myself. How do I know which O2 sensor is bad, the one in the manifold or the one by the cat?

I have a 97 voyager 2.4 motor. I am getting bad gas milage. The check engine light is giving me code 21. I want to fix it myself. How do I know which O2 sensor is bad, the one in the manifold or the one by the cat?

Codes do not say replace sensors. They say check things out.
Code 21 can be 4 different problems on the downstream 02 sensor or wiring circuit.
Check all fuses to start with.
You will need the 4 digit P---- codes to know where to check.
Get a good scanner on it and read out data and codes and post back code no.
MT

Code 21 can be either the upstream (manifold) or downstream (cat) sensor according to the Haynes manual. As MT says you'll need a scanner to determine which one. I think Autozone has scanners that will read it and they do it free.

I have a 97 voyager 2.4 motor. I am getting bad gas milage. The check engine light is giving me code 21. I want to fix it myself. How do I know which O2 sensor is bad, the one in the manifold or the one by the cat?

As RIP said autozone do give free code checks.
BUT.
Use Caution when getting the free code checks please.
Some parts places do offer free code checks. And also free repair advice. But beware. Just get the code no and post it back here and someone will help you.
They offer the free code checks in the hope of increasing there parts sales.
Almost all of them will also offer mechanical advice and some know what they are doing.
But just remember most of them are parts people pulling codes for free.
Not trained professional tech/mechanics. Even if they pull a 02-sensor code or another sensor related code it does always mean to replace that sensor and your problem will be fixed.
Please just get the code no and post it back here and we will help you.

Good Luck MT

Thanks I will get it checked this weekend. I will post back.

I replaced my O2 sensor and it fixed the problem.

What was your code #? If you turn the key switch on and off 3 times then leave it on you will get flashing code too.
Just curious its not as accurate as a code reader.
Thanks