I was driving along today and my Check Engine light came on. I stopped at the nearest Advance Auto to have them pull the code and it came up with the P0420, Catalyst Efficiency Below Threshold (Bank 1). I asked what could cause this and they mentioned that the fuel that was put in could be causing it. They didn't mention any other possible causes. Any others? I'm thinking it could be the fuel because the last gas station that I stopped at and filled up at is one that I never go to, and maybe just got a bad batch of fuel.

 

it could be bad gas, but could be a failing cat too. run it low on gas, fill up where you normally do. disconnect the battery cables when you get home, reconnect after 30 seconds to clear the code. if it comes back your cat is failing... its up to you what you want to do, if you have under 80K on it the part is covered but the labor is not.

 

I am a technician and I'll tell you the only 2 things I have seen cause a P0420 is abad cat or a bad O2 sensor. The O2 sensor can trick the computer into thinking the cat is not working, but 9 out of 10 times it is the cat.
Brad

 

How do I tell if it's the O2 sensor or the cat? The truck has 98k miles on it.

 

The only way to know for sure is to monitor the O2 sensor with a scan tool. Unfortunately the dealer is one of the few places that can do that. But I would say it is likely (95% chance) the cat.

 

Great. That's all I need is to have to replace the cat. I'll go through this tank of gas, go to my usual fill up station and have the code reset, see if it comes back. I'm hoping it's just a bad batch of fuel.

 

I agree with bhsuperduty, being a tech also 95% of the time it's the cat.

 

It's the cat. Both O2 sensors on that bank have to be working properly for the PCM to recognize that the cat isn't working.

 

On general how much is a new cat installed? And while it's being replaced would it benefit me to have high flow cats installed? But COULD it still possibly be the gas that was put in? I'm still holding out hope that it could be the gas.

 

Depends on which side it is. If you go factory it is going to costs some bucks. There is an aftermarket source for the cat assembly. It is considerably cheaper but I haven't heard any reviews of it. Your cats are already high flow.

As far as having them installed it's a pretty straightforward job. Might as well change out the O2 sensors while doing it. I'm looking at doing a manifold back system before too long and that is what I'll do.

 

I know for a fact I won't take it to Ford to have them do it. If any it's something straight forward and not too difficult I'll go buy the cat and my uncle and I can do it. Now, if I'm replacing one already would it be beneficial to go ahead and replace the other as well? Also by going aftermarket will it help the sound of my exhaust any? If I'm replacing them might as well put in one(s) that'll help and work great instead of putting new ones in just to replace them again with better ones within a year.

 

I plan to replace the cats and install a cat back system when I do it since my muffler is starting to split. Too many high rev's through it.

 

Alrighty, just thought I would ask about the exhaust sound just in case. I've been driving the truck the past few days with this problem and it hasn't really effected anything and it's not a pain in the rear to drive. Would it hurt it much to continue driving it like this until I'm able to get it tested to make sure it's the cat? I do plan on taking a trip to Kansas over memorial weekend to pick up my Harley, would that not be a good idea with this problem or would it be okay? I do plan on getting it fixed, whatever the problem, just can't afford it at this particular moment.

 

Anything is possible, but in the real world no. The way the PCM measures catalyst efficiency is by comparing the readings of the O2 sensors. It is expecting the upstream to be a sine wave between roughly .2 and .9 volts. It expects the downstream to be nearly flat lined somewhere in between. It sets the code when the downstream reading starts mimicing the upstream reading. I can't think of any way that bad gas could cause that to happen without setting other codes first.

 

Alright, bad gas was something that both Advance Auto and Auto Zone both told me that it could be. Is there a way other than taking the truck to the dealership that I can tell if it's the cat, something I can do on my own at home?