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Old 01-17-2011, 11:09 AM   #1
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Question P0420 code on Astro

I have a 2002 Astro 2WD with 137k on it, original owner. The past few days it'* been throwing a P0420 code, "Catalyst System Efficiency Below Threshold (Bank 1)." I have an OBD II reader and have been resetting the code, but it'* come back on now 3 times.

Does anyone know what exactly would trip this particular code? I had the cat replaced less than 20k ago due to an unrelated problem. The 3-stud exhaust flange at the rear end of the cat is missing one of its studs (broke off), and I haven't gotten around to drilling out the remains and replacing with a nut and bolt yet as it'* freakin cold out and snowy. Would the (very minor) leak that'* resulting from the missing stud cause this code?

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Old 01-17-2011, 11:42 AM   #2
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sounds like the front o2 sensor
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Old 01-17-2011, 12:11 PM   #3
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I initially thought it might be an O2 sensor, but the Bank 1 codes are P0130 through P0147. Would rather not invest a hundred bucks on a sensor until I'm sure that'* what it needs....

I've replaced the downstream O2 sensor, at about 100k miles.

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Old 01-17-2011, 11:17 PM   #4
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Unfortunately, this code either means that the converter has stopped converting, or the O2 sensors are too worn out to be accurate while having not failed completely.

RockAuto has Standard Motor Products' sensors priced from a very comfortable $30 (for the Bonneville; your make/model/year will vary). I'm not familiar with that vendor, so do your homework or pick another brand.
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Old 01-18-2011, 10:16 PM   #5
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Here'* the information I have on the P0420 code. https://www.gmforum.com/showthread.p...86#post1390979
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