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  #1  
Old 11-25-2017, 02:58 PM
lisjcas lisjcas is offline
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2008 Chevy TrailBlazer SS (3SS)
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Persistent check engine light.

I have an 08 TBSS with 63,000 miles. The check engine light has been coming on for the past month and a half. We've taken it to the local Chevy dealer, 2 other mechanics and a performance exhaust place. My husband has a code reader, and the code always comes up P0420. The bank 1 cat has been changed to a Magnaflow cat. Both of the bank 1 sensors have been replaced. A vacuum hose on bank 1 has also been changed. The longest the check engine light has remained off was 3 days. It was worked on this past Wednesday. The check engine light came back on this afternoon. Has anyone had a similar problem?
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  #2  
Old 11-25-2017, 03:58 PM
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EnvoyBoy2004 EnvoyBoy2004 is offline
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Did any of these "experts" check the downstream O2 sensor voltage using a scan tool. That sensor should maintain a fairly steady voltage reading if the cat is doing its job.

It may be possible that the wiring from the ECM to the sensor is faulty.
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Old 11-25-2017, 06:15 PM
lisjcas lisjcas is offline
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2008 Chevy TrailBlazer SS (3SS)
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Engine light.

I don't know if they're "experts," but they're all ASE certified. Thanks.
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Old 12-10-2017, 09:30 AM
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TB-Al TB-Al is offline
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A code P0420 may mean that one or more of the following has happened:
•Leaded fuel was used where unleaded was called for (unlikely)
•A damaged or failed oxygen / O2 sensor
•Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
•The engine coolant temperature sensor is not working properly
•Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
•Failed or underperforming catalytic converter (likely)
•Retarded spark timing
•The oxygen sensors in front and behind the converter are reporting too similar of readings
•Leaking fuel injector or high fuel pressure
•Cylinder misfire
•Oil contamination


Also you say you installed a new converter. Code P0420 may set due to outgassing of the mat material with a new converter. You need to drive at highway speeds for an hour to "break-in" a new converter. Finally, the reason the code cleared and then came back 3 days later might be that the system say one clear functioning converter and then it went "bad" again. The light does not come on immediately when a problem is detected. It can take 3 driving cycles for the light to come on. But you would see a pending code with a code reader.
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Old 12-10-2017, 01:24 PM
lisjcas lisjcas is offline
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2008 Chevy TrailBlazer SS (3SS)
Bordeaux Red Metallic 6.0L LS2 V8 AWD
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P0420

Hi,
Thanks for the info! Good to know about driving for a while after. I had new spark plugs and wires done last week. The converter was replaced (again) last week. So far so good.
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