Service Engine Code

**BajaMark** · 07-02-2008, 08:42 PM

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
Mark

**GaryL** · 07-03-2008, 10:06 AM

This may answer some of your questions:

http://www.obd-codes.com/p0420

**MrShorty** · 07-03-2008, 11:17 AM

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.

**BajaMark** · 07-06-2008, 01:42 AM

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

**BajaMark** · 07-06-2008, 01:43 AM

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

**FireRed** · 07-06-2008, 01:21 PM

Check this out you may be covered, if so take it to the dealer and save your blood and sweat!!

http://www.epa.gov/oms/consumer/warr95fs.txt

**MrShorty** · 07-06-2008, 04:20 PM

Originally Posted by BajaMark

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

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?

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

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).

**BajaMark** · 07-07-2008, 11:35 AM

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?

**MrShorty** · 07-07-2008, 02:10 PM

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.

**BajaMark** · 07-08-2008, 01:28 AM

Originally Posted by MrShorty

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.

Thanks a lot for the help