Before I say anything I will mentioned I have searched for the past few weeks and continue to get mixed results. I have been getting so many different opinions that I need to finally ask the question myself.

So I bought my 2006 stock scion tc last may. Was getting 350 miles to the fill up. About a month or 2 later I got the P0420 Catalyst system efficiency below threshold bank 1 code. First things first I cleared and and of course came back. Now I have had 0 issues as far as running or anything that was showing that something was wrong except this engine light. Made a drive 900 miles with no issues. Now as time has gone by my MPG has seemed to go down and lately its been 300 miles to the fill up if not a little less. Same engine code is on. Again no running issues, but starting to think a bad or plugged up cat. I am really concerned because loosing MPG is a sure sign something is wrong. Since I do mainly high way anyways I figured I should get around 375 to a fill up and not 290-300.

So from my research I saw this could be: Bad 02 censor, plugged cat.

I have also seen my options as being: Replace cat all in itself which would be very expensive, replace header, get a new exhaust, punch our cat converter and get a 02 censor spacer.

I keep getting thrown around what I should do. I am a college student and not looking to spend a fortune, but will spend whats needed to fix the issue. What do you guys recommend I do? I have been a express mechanic for 6 years so I have good knowledge about working on cars just not with exhaust.

Lastly could this be what is causing my gas mileage drop?

 

Come on guys... really? 104 views and you guys got nothing for this guy? That's upsetting...

Express mechanic huh? You do lube and filter type stuff? That's more advanced than exhaust, I'll tell ya that much. If you have the right tools, there is no need to spend any money on labor. Now, it is definitely throughing that code because of your O2 sensor or because of some sort of build up. Your ECU is finding a problem with the levels and that is why your MPG is down, your ECU is upset with the situation and is trying to use more fuel to counter the problem. Now, the thing is is that you could have a problem with your header cat or your secondary cat. You'll have to find which one is the problem. Second, what I would do is go under your car and grab a wrench and make sure both your O2 sensors aren't loose and make sure there are no cut or shorting wires coming from the sensor. After that, you are going to need to cut out your cats and weld in some $20 piece of exhaust piping. Or can you buy cats and weld those in too depending on your counties emission laws. If there aren't any, go with standard piping. You'll get a deeper tone as well. If you have a friend or someone of your express mechanic buddies that have these tools, you're golden. If you have nothing and no resources, then yes, labor is not going to be fun. If there is build up on the inside of your cat by the O2 bung, yeah, you're gonna have to replace it. There's no real way of telling, but if you got the tools, its an afternoon worth of work. Let me know if you have any other questions once you figure out a little more.

 

Actually have more experience then express just wasn't going to jump to conclusions that exhaust work was easy since I have never touched one. Your post has been very helpful because I now have a starting point. I just wanted to know what my options were as far as things go if it is the cat. Honestly I would like to do away with it and get a header that removes it that way I dont run into another issue with it. That being said I know that I would have to do something about the sensors so I wont have the engine light on. Being a mechanic making sure my car is well taken care of is very important to me.

I do of course have lots of resources as far as mechanic friends, technicians, hell I even worked at a machine shop and have welders that could be of help if needed. I do intend to do all labor myself, but just need to make sure I can gather as much knowledge as possible.

Also I do appreciate your reply it was bugging me so many views and nothing at all.

 

Very cool. I'm glad you appreciated my post and found it helpful. I felt like I actually didn't have enough to say honestly. But yeah. Especially if you have the resources, you are golden. Dude, if you wanna do it right and get rid of all stock exhaust from the head to the tip, yeah, go that route. It won't cost you nothing. Get a used aftermarket header off someone here who's selling one. you can get them for like 75-100 dollars for some of the no-name ones. Otherwise you can spend 150 buck on a used name brand one. People are willing to sell those all day for nothing. Headers are cheap and they don't have the cat for you to worry about. You will need a CEL delete but that's another story you can ask me later once you get to that point. And then for exhaust, you can fricken make one if you have the resources, you just need a tube bender and a welder and a days time and your good. You just need like $100 worth of exhaust piping. Otherwise, you can probably pick up an aftermaket exhaust off this forum for like 200 or so dollars for a decent one. But at a mininum get a reciprocating saw and a welder and a two feet of exhaust piping and you are golden. Anyways, glad you liked my post and found it helpful. I've done tons of exhaust jobs, if you have anymore questions let me know and let me know how your journey is with fixing the problem.

 

What ever I do I wanna make sure I accomplish a few things. First off has to be cheap. Still a college kid that doesnt make very much. Second I dont want my car sounding loud lol. Third I want it to be a fairly easy even if its long and reliable. Which its sounding like getting a new header to do away with that cat will be a good option. Thats a win win situation because even if my cat isnt bad, from what I hear they usually go out at some point.

The first thing I am going to do though is pinpoint my problem. I have some guys that I should be able to talk to this week that know more about exhaust and have ways of checking them out. When we can pinpoint the issue I will post on here.