bad cats or o2's??

[elwood_]

I recently purchased an 05 sierra 1500 single cab with 269k miles. It was a fleet truck so i got it really cheap and it was fairly maintained. When i got it a little over a month ago the SES lights was on for low cat efficiency on bank 1, i reset it and it came back within a few days and it's been like that since. Today i checked again and now bank 2 is on also.

 

I'm assuming because of the high mileage they are just plain clogged. Coolant levels have not changed and neither has the engine oil so those two are ruled out as the possible cause.

 

I've decided the best and one of the cheapest routes is to get two Magnaflow 99006HM universal cats and have an exhaust shop cut my old ones and weld the new ones in.

 

Before i do that i'd like to change the pre-cat o2 sensors and here's where the confusion starts. First, i'm either gonna go with Denso or AC Delco sensors. AC delco shows a few different sensors for upstream at significantly different prices. Anyone know the difference between the AC Delco 213-3867 and 2131148?

 

Before having the cats replaced i will replace the front o2's, plugs and wires, air filter, and clean the MAF. Any other suggestions?

 

Also, what is the odds that both rear sensors are bad? I haven't had the truck for long but there doesn't seem to be a loss of power, average fuel economy, and no bad exhaust smell. Should i try cleaning or replacing the rear o2's first?

 

Thanks!

[aflesch]

I am an ASE Master Tech and I have never seen a lone Low Cat Efficiency code caused by anything other than a bad cat. If the o2 sensor is bad it will throw a different code (possibly along with the Low Cat Efficiency code). Just my experience.

[elwood_]

I am an ASE Master Tech and I have never seen a lone Low Cat Efficiency code caused by anything other than a bad cat. If the o2 sensor is bad it will throw a different code (possibly along with the Low Cat Efficiency code). Just my experience.

 

 

Thanks! So i order two cats and have them replaced. I replace plugs, wires, air filter, and clean the MAF (needs all this reguardless) anything else i should look to change or clean?

[aflesch]

I would replace the fuel filter and clean the throttle body also.

[elwood_]

I would replace the fuel filter and clean the throttle body also.

 

I'll clean the throttle body but the fuel filter is in the gas tank so i think i'll leave that one out.

 

thanks

[txab]

I am an ASE Master Tech and I have never seen a lone Low Cat Efficiency code caused by anything other than a bad cat. If the o2 sensor is bad it will throw a different code (possibly along with the Low Cat Efficiency code). Just my experience.

 

 

I have seen low eff. codes thrown without a cat being bad. Short trips and poor fuel quality can cause it. Had two different vehicles throw codes for one bank or the other. Found out they were getting fuel from some mom and pop station that buys the cheapest crap they can get. Changed to a top tier fuel and started driving a bit more (longer trips), including a little highway trip. The problem went away. These were vehicles with < than 100k miles.

 

Yes we thoroughly checked the cats condition and they were fine.

 

With your mileage, your cats are probably toast. The rear O2s have nothing to do with fuel efficiency.

[elwood_]

I rescanned the codes again today and it's only saying bank 1 cat low efficiency so i must've been seeing things when i said it was both banks (i never reset it).

 

I logged the o2's on my way home from work, let me know what you guys think.

 

http://minus.com/l7Cv8sLz3Wdqm

[aflesch]

That looks like both Cats are bad. The Post-Cat O2 sensors should bounce back and forth between .6 and .8 volts with an occasional drop when you first step on the throttle. They are dropping WAY down A LOT! Either you have an exhaust leak between the Pre-Cat and Post-Cat O2 sensors that is fooling the PCM to think the cats are bad on both banks, or you have 2 bad cats. I would say it could be a lean condition causing the code but that should set p0171 & p0174 codes (lean on both banks). With as many miles as you have on the vehicle I would put my money on the cats.