Ninja11

As i was driving today i got a check engine light so i went to advance auto and got them to run a test. It turns out that the cat is going bad (thats what they said). then my dad called Dan Driskill and he said to try changing the o2 sensors first. Tomorrow were going to go get it tested somewhere else (and have them actually print out what is wrong instead just saying theres a catalytic problem) and buy the o2 sensors. anyway so i was wondering if there is any way to tell which o2 sensor it is so i dont have to replace both of them?

Fabrik8

I'm assuming Advance just hooked an OBD2 scanner up to it and read the codes?

It's a hard thing to narrow down sometimes. The ECU compares the front and rear O2 sensors, which are obviously on each side of the cat, and uses the readings to check if the cat is within proper operating spec. One of the O2 sensors is a wideband unit usually, and one is a cheaper narrowband. The wideband is usually used for most engine operation, and if that O2 sensor goes bad you will generally have problems with the engine running well. So if the engine is running fine, and you're getting a cat efficiency CEL or whatever, replace the narrowband (rear I'm pretty sure, but check just in case) first because it's cheaper. I don't generally like to recommend blindly replacing parts to try to solve a problem, but O2 sensors aren't something that can be easily tested because of the heating elements and the fact that they frequently fail slightly (as opposed to completely).

Ninja11

ok thanks. the guy just used the little handheld theng. Tomorrow im goning to go and see if they can use the biger computer thing and print out the diagnostic. either way im gonna end up buying the o2 sensors. ill go w/ the cheaper 1 first lol.