Service Engine Code

Discussion in 'General Chevy & GM Tech Questions' started by BajaMark, Jul 2, 2008.

  1. BajaMark

    BajaMark Member

    Hey guys I have a 99 Silverado with 115,000 miles on it. I recently got a service engine code displayed on my dash. I looked up the code and it came back with PO420 which said Catalyst System Efficiency Below Threshold (Bank 1). Any ideas on what needs to be replaced or fixed. Im guessing O2 sensor, but im not sure.

    Thanks for the help
  2. GaryL

    GaryL Epic Member 5+ Years ROTM Winner 1000 Posts Platinum Contributor

  3. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    I just went through the same thing (only a P0430 for the other bank).

    1st thing I did, because I have a cat for each bank, was to switch the two post cat O2 sensors to see if the code followed the sensor or not. If you have two cats, I'd probably suggest this as a first step. If the code follows the sensor, the sensor's probably bad. If the code doesn't, then it's something else.
  4. BajaMark

    BajaMark Member

    Is there two O2 sensors in a 99 silverado, and what would be the easiest way to test which one is the bad one?
  5. BajaMark

    BajaMark Member

    If I switch the two banks, will a different code number pop up
  6. FireRed

    FireRed Rockstar 100 Posts

  7. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    I know my 5.7 has two cats and, therefore, two post-cat sensors. I can't say for sure for the other engines. Which engine do you have?

    Only if the fault is in the O2 sensor itself. If the code stays the same, the fault is not the sensor (which means either the wiring between the sensor and the PCM or the cat itself).
  8. BajaMark

    BajaMark Member

    Its the 5.3L. Ive heard there is an o2 sensor before and after the cat. Im confused is there only one 02 sensor per cat, or is there two 02 sensors for each cat?
  9. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    I believe there are usually 2 O2 sensors per bank (ie per side of a V engine). The O2 sensor upstream of the cat monitors exhaust gas oxygen and sets up a feedback loop to optimize fuel/air mixture in the engine. The O2 sensor downstream of the cat monitors oxygen levels to see if the cat is functioning correctly. The cat efficiency codes involve the post-cat O2 sensors, so those are the ones you need to focus. This is true if your 5.3 exhaust is plumbed like my 5.7 exhaust. OBD-2 systems ever join the 2 exhaust streams upstream of the cat, in which case you'd only have one catalytic converter and one post-cat O2 sensor.

    The sensors I'm recommending swapping are the two post cat O2 sensors, if you have two cats.
  10. BajaMark

    BajaMark Member

    Thanks a lot for the help

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