Which ones first?

[McMuscle3]

My trucks mpgs have been sucking here lately. I replaced the fuel filter, but that didnt phase it. I was wondering if what I should replace next? 02 sensors or cats? I would replace the cats with these off ebay.http://www.ebay.com/itm/150861262214?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
My engine light is on for some code but I cant rememebr what it was. I think it was the 02 sensor downstream cat or something.
12.8mpg hiway! 265 75 16 tires, not letting the rpms above 2k.
What you guys think?

 

[SkyHighColorado]

I would replace the o2 sensors. If you have over 200k then I would also replace the cat, or if it is rattling or anything like that. Check your tire pressure, higher pressure gets you better mpg's. Replace your spark plugs with ac delco's if they are old. I would also take off your intake and watch the intectors spray and see if its even.

 

[McMuscle3]

Thanks, Should I replace all 4 sensors? I have dual cats so Im assuming I have 4. I havent got up under there and checked though.

 

[SkyHighColorado]

Thanks, Should I replace all 4 sensors? I have dual cats so Im assuming I have 4. I havent got up under there and checked though.

What year is your truck? I think you will have 2 of them. one on the manifold and one right before the cat. I would replace both of them, but i think you can hook up a computer and it will tell you what each sensor is doing. If the numbers it is reading are steady then they have failed. dont quote me on that though.

 

[McMuscle3]

98 gmc sierra 5.7l. I know for sure I have two cats because I have true duals from headers back.

 

[McMuscle3]

rock auto part # sg272 and sg454. I would need two of each, is this the right part?

 

[McMuscle3]

if its the p0430 that would mean my upper 02 sensors are good right? so i would need maybe 2 lower ones which one would part number sg272 @ rock auto.com,,,, the scanner said catalyst system deficiency below bank 2 (p0430)

 

[SkyHighColorado]

I think those are the right ones, but I would let someone who has a vortec truck chime in.

 

[McMuscle3]

Yeah, Well when I did the truck search it shows up. so im pretty sure they will be. I may order them and spark plugs tonight or tomorrow. It will be cheaper then the cats, plus it needs done anyway. lol

 

[bluex]

The post cat O2 sensors have no effect on fuel economy at all. There only job is to check that the converters are working properly.

Yes you have four sensors, bank 2 is the passenger side.

If you don't have emissions testing try replacing the converters with a piece of pipe an see if that helps.