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1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

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Old 09-20-2007, 08:59 AM   #1
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1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

I was getting P0141 (O2 Heater Circuit Malfunction (Bank 1 Sensor 2)). So I replaced the O2 sensor about a year ago. That cleared the code immediatly. Now I keep getting P0420 codes.

I see that P0420 is Catalyst System Efficiency Below Threshold and "A code P0420 may mean that one or more of the following has happened":
  • Leaded fuel was used where unleaded was called for (doubtful)
  • An oxygen sensor is not reading (functioning) properly (possibly bad sensor again? - i'd expect the P0141 code again)
  • The engine coolant temperature sensor is not working properly (this looked good)
  • Damaged exhaust manifold / catalytic converter / exhaust pipe (cat is not clogged?)
  • Retarded spark timing (i have no idea)
What should I look at next? The cat was not clogged and could see right though it. I've replaced both pre/post cat O2 sensors ~ a year ago.

Inspection is coming due in a few months..... It's a pain b/c it takes ~200 miles for the code to come back!

any help??!!! thanks!
Old 09-20-2007, 07:00 PM   #2
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

sounds like o2 sensor...again
Old 09-20-2007, 07:39 PM   #3
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

that code means your cat has failed. do u have a scantool or know anyone with a scantool that can read sensor data? if you see the post cat o2 sensor fluctuating like the pre-cat o2 sensor then you know your cat has failed. try this . run your engine until its at optimum temp. unplug the post o2 sensor and probe the signal wire with a multimeter(multimeter only and it has to be set to ac current). you should see a steady voltage if the cat is good. if the cat has failed the voltage will fluctuate from rich to ean or 0v-1v
Old 09-20-2007, 08:28 PM   #4
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

I have a Actron CP9135, I'm not sure if it can read that data however.

I believe the sensor is 3 wires, what do I probe? Unplug from the truck and probe the sensor?
Old 09-20-2007, 08:29 PM   #5
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

Oh and 1 more thing, can the cat be bad even if it's not plugged up?

thanks for the replies!
Old 09-20-2007, 08:56 PM   #6
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

Quote: Originally Posted by nicks10
Oh and 1 more thing, can the cat be bad even if it's not plugged up?

thanks for the replies!
theres a catalyst inside, when all that burns up , yes it can go "bad"
Old 09-20-2007, 10:10 PM   #7
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

Quote: Originally Posted by nicks10
Oh and 1 more thing, can the cat be bad even if it's not plugged up?

thanks for the replies!

yes, they can still go bad even though there not plugged up. unplug the connector and probe the odd color wire. ex: grey, tan, brown. if there are 3 wires then you should see a black, white, and odd color wire. look under and tell me what colors you have.
Old 09-21-2007, 07:10 AM   #8
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

Quote: Originally Posted by lampug
yes, they can still go bad even though there not plugged up. unplug the connector and probe the odd color wire. ex: grey, tan, brown. if there are 3 wires then you should see a black, white, and odd color wire. look under and tell me what colors you have.
Sorry, I thought it had 3, it actually has 4 wires.

From the vehicle, it has black, purple/white, brown, and brown/white

On the sensor it self it has white, white, black, and grey
Old 09-21-2007, 12:29 PM   #9
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

Quote: Originally Posted by lampug
that code means your cat has failed.
+1
Old 09-21-2007, 05:43 PM   #10
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

you need a cat
Old 09-21-2007, 05:49 PM   #11
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

mine also keeps throwing that code both my o2 sensors are new. i gutted the cat is that why my light keeps coming on?
Old 09-23-2007, 06:56 PM   #12
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

I gutted my cat also, my state doesnt require emissions, I need to know what to do to correct this code, would a simulator be the answer? any answers suggestions would be helpful
Old 09-23-2007, 06:57 PM   #13
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

mine is a 2003 sonoma 4.3L
Old 09-23-2007, 09:07 PM   #14
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

You most likely need a cat but also remember to use only a bosh direct fit or a factory 02 sensor. The univversal 02 are junk.
Old 09-24-2007, 12:52 AM   #15
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

good luck findin an o2 sim...caspers stopped making them
Old 09-24-2007, 08:13 AM   #16
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

I'm going to try the sensors again before going with the cat. Replaced the rear (post) O2 sensor. Now just have to wait 200+ miles.....
Old 09-24-2007, 04:10 PM   #17
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

My 01 cat is gutted and the CEL has not come on for that yet. It is on because I broke the wires on the pre-cat O2. I ran it yesterday and it said only pre-cat faults.
Old 10-01-2007, 08:58 PM   #18
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Re: 1998 s10 keeps throwing P0420 Catalyst System Efficiency Below Threshold

Well it's not the sensors, I replaced them under Advance Auto's 1year warranty.

The cat was an aftermarket one, replaced it about 5 years ago. I've ordered a new cat ($130). I'm guessing that's it.
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