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I had a check engine light indicating faulty.

Tiny
ANONYMOUS
  • MEMBER
  • 2001 TOYOTA SIENNA
  • 142,000 MILES
I had a check engine light indicating faulty catalytic converter, O2 sensor or other emissions related problem. The shop said the catalytic converter and o2 sensor was faulty so I replaced both, using a universal catalytic converter. After a week, the check engine light came back on and has stayed on, with the same code as before. I took the car in for an emissions test, which it passed with flying colors despite the light remaining on with the original fault code.

why do I get this check engine light and code when it passed the emissions test and what do you suggest I do to get rid of the light?.
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Tuesday, October 30th, 2012 AT 4:19 PM

7 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
It can be a bunch of things, when you replaced the converter, it may have an air leak causing the converter to not heat up correctly. One or both of the O2 sensors may not be reading incorrectly as well. Or it cold be other thingstoo.
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Tuesday, October 30th, 2012 AT 4:31 PM
Tiny
ICANDOITDON
  • MEMBER
  • 2 POSTS
You did not mention the possibility of a false reading, triggering the check engine light.

Why should I assume anything is wrong if it passed the emissions test?
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Tuesday, October 30th, 2012 AT 4:35 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Normally false readings occur with a bad battery and yu asked why does it light, I just gave you some thing why it would
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Tuesday, October 30th, 2012 AT 5:11 PM
Tiny
KHLOW2008
  • MECHANIC
  • 41,814 POSTS
What are the codes you are getting?
The EXACT codes are required.
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Wednesday, October 31st, 2012 AT 8:59 AM
Tiny
RIVERMIKERAT
  • MECHANIC
  • 6,110 POSTS
Icandoitdon, we'll need ALL of the codes. You'll need a quality code reader to read the soft codes as well as the hard codes. You should also check the wiring and connectors to the O2 sensor(s).
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Wednesday, October 31st, 2012 AT 10:40 PM
Tiny
ICANDOITDON
  • MEMBER
  • 2 POSTS
The error was diagnosed as P0420, "Catalyst system efficiency below threshold - bank 1." I replaced the catalytic converter and O2 oxygen sensor and it hasn't helped. Wondering if the air flow sensor might be the problem. Haven't replaced that yet.

What makes me suspicious is that the car passed the emissions test with good readings on everything even though the light was on due to thiis code (p0420).

Any other thoughts would be appreciated.
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Thursday, November 1st, 2012 AT 2:18 AM
Tiny
RIVERMIKERAT
  • MECHANIC
  • 6,110 POSTS
Check the voltage to and from the O2 sensor(s) on Bank 1. Let us know what you get.
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Thursday, November 1st, 2012 AT 1:45 PM

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