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  1. #1

    Default Bad cats and o2 sensor confusion, please help!

    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?


  2. #2


    Recommend testing them first. The O2 sensors above the cats should oscillate above and below .45 volts indicating the computer is adjusting the fuel/air ration to rapidly compensate for too much, too lean (it's never "perfect"). You'll get very slight variations in oxygen output, and the oscillations indicate your O2 sensor is measuring the changes being made. After the Cat, the O2 sensors should basically be flatlined, indicating the catalytic convertors are functional.

    If you've lost the convertors, be sure you diagnose the reason why FIRST. Too rich conditions resulting from injectors over-supplying fuel, or leaking into the combustion chambers will result in exhaust gasses filled with fuel. The unburnt fuel hits the cats and ignites at 1400+ degrees burning out the catalysts. Replacing convertors without addressing the underlying reason will result in more burned out convertors.

    Bad O2 sensors can also contribute to this. As for which O2 sensors your engine needs, I'm unsure the differences. If you can find a computer that pulls live data (many handhelds will do this), use it to look at bank 1 and 2 O2 sensors and the second sensors (downstream from the cat) and ensure O2 response is correct. No point in replacing functional O2 sensors. Often you'll hear of "sluggish" sensors, that'll show up in the charting as well. Basically they won't oscillate frequently, indicating the O2 sensors aren't responding rapidly enough to the changes made by the computer (resulting in significantly rich conditions and significantly lean conditions before notification occurs (which is then reported as a barely rich or lean condition).

    If your rear O2s are flatlined (showing voltage, but not oscillating) like they're supposed to be, your Cats are functioning. If you're getting NO feedback, the O2 sensor is dead.
    Hope this helps.

    2006 Vortec Max 1500
    -Custom Tune (389HP 440 Ft/lbs Trq - Dyno'd) | Volant CAI | Magnaflow Dual-in/out Exhaust w/3" stainless pipe tips | Fully Built Transmission w/ Red Eagle Clutches & Kolene Steels w/ Corvette Servos and Stage2 Shift Kit | 35K Tranny Cooler | Mobil 1 | Royal Purple Rear Diff
    -Spray-in Bed Liner
    -Premium Sound w/lifetime Satellite Radio | Leather | Sunroof | Heated Seats
    -Limbstriping from USING the truck (those are badges of honor)

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