Any idea what's causing it to throw this code. Truck seems to be running fine but the check engine light came on.

 

Yeah your rear O2 sensor says your catalytic converter is below efficiency.

You can clear that code and see if it comes back, sometimes it was on the edge and you won't get it again. One of my older cars it gets it every 6 months and I clear it, and I don't bother. You need that cleared and the computer done with it's reset though to pass emissions.

If you look up causes dirty injectors and other things can cause it, along with a bad cat. I'd do a search.

 

Thanks I appreciate it. I actually just put a bottle of techron fuel system cleaner in it before I filled up yesterday so maybe that'll take care of it. I went ahead and cleared it out so hopefully it won't come back.

 

That type of catalyst efficiency related code is usually a warning sign that the catalytic converter is getting towards the end of it's lifespan or there's possible an exhaust leak. As another member mentioned sometimes the code can go away once cleared if the exhaust is in good shape up front and the cat's doing it's job fully. Other times if it's really coming on for a legitimate reason it'll come back within 50-100 miles of driving once it sees the issue is still present. I have a 2001 Expedition that was throwing the same code along with 1 or 2 similar codes, and I found the cat / manifold connection pipes were rusting out pretty bad and had some holes so the code was legit in my case. I had the exhaust repaired and the codes haven't come back since. So if you haven't already., it would definitely be a good idea to check the exhaust from the manifolds to the rear o2 sensors to see if there are any issues or leaks that could explain the code.

 

clear it and see if it comes back. as mentioned above if it comes back within the next 100 miles there's probably an issue within the exhaust

 

yeah if it throws it out again I'll crawl up under there and take a look.

 

ALL DTC's are 'threshold' triggered. As has been mentioned here, you could clear it an it may not come back. But other systems that effect the Catalyst operation may be out of specification - yet below THEIR threshold trigger and have not YET thrown a code.

Your CAT converter has to get rid of 'excess hydrocarbons' that comes it way by chemically burning them off. Running a little 'richer' fuel/Air mixture for an extended period (But below triggering LEAN code) causes CATS to work much harder ----- and VERY possibly DTC trigger level above their ability to burn off the excess hydrocarbons.

I would check for other signals above or below their norms in conjunction with clearing your code.

IE:
Intermittent misfires on Bank 1 (Below Misfire DTC level) throws excess fuel into exhaust an overtaxing the CAT over a period. Check Mode 06 reports for misfire counts over last 10 drive cycles. (could be as simple as needing plug job / tune up).


Small vacuum leak (Below LEAN CODE DTC threshold) can result in elevated Fuel Trim numbers on STFT1. Actually this (or MAF) could be effecting both banks - just that CAT two can handle the overload better than CAT 1. But Fuel trims are a very useful indicator of what's going on. Graphing Fuel Trims an O2 voltages on both banks can be very useful.

Elevated fuel trims on bank 1 would also show up if you had a small exhaust lean on Bank 1 like @Livernois Motorsports suggested - just would be a confirming indicator.

There are other readings that might provide additional input. The above are first couple that come to mind. As always, DTC's are many times just one of many clues and seldom specify part that needs to be replaced or fixed.