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I am curious - whats the thinking behind a 2nd re-test? If it failed on first attempt and nothing has changed in the moment, why re-test? Why would someone expect a different result?
 

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Mopartial to Challengers
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9,140 Posts
I am curious - whats the thinking behind a 2nd re-test? If it failed on first attempt and nothing has changed in the moment, why re-test? Why would someone expect a different result?
It could be something as simple as a fresh oil change or replacing an aftermarket gas cap with the stock, OEM cap. Temperature might also make a difference as smog station mechanics have advised me in the past to arrive right at my appointment time since an engine (off a fresh run) will give better emissions test results than a car that has sat for 30 minutes or longer. I don't know if this is true or not, but I owned a Civic for 17 years and 300k miles that never failed an emissions test until the final one.

And then the State of California offered me $1000 for it through a program we have here in the Golden State to ensure that gross polluters are pulled from the road. I made another $500 off the old Civic stripping interior parts and re-selling them on eBay before I surrendered my car to the salvage yard ;)
 

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Dash Cam Guy
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The first test is OBD-II which is easily failed with any DTC lit up. The second test would be a sniffer test where trouble codes are irrelevant. Let's say you have a small exhaust leak. You will fail the OBD-II test because of the engine light. The sniffer test on the other hand is only concerned with the makeup of the exhaust leaving your tail pipes. As long as your cat converter is working you should easily pass the sniffer test.
 

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Dash Cam Guy
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2,277 Posts
It could be something as simple as a fresh oil change or replacing an aftermarket gas cap with the stock, OEM cap. Temperature might also make a difference as smog station mechanics have advised me in the past to arrive right at my appointment time since an engine (off a fresh run) will give better emissions test results than a car that has sat for 30 minutes or longer. I don't know if this is true or not, but I owned a Civic for 17 years and 300k miles that never failed an emissions test until the final one.

And then the State of California offered me $1000 for it through a program we have here in the Golden State to ensure that gross polluters are pulled from the road. I made another $500 off the old Civic stripping interior parts and re-selling them on eBay before I surrendered my car to the salvage yard ;)
When you cold start your car it runs in open loop fuel mode. In this mode it determines the fuel requirement largely based on coolant temperature. This mode will usually result in a richer fuel mixture which as the smog guy said can affect the smog test. After the car warms up it enters closed loop fuel mode. Once in closed loop it primarily uses the upstream o2 sensor to control the fueling and results in a more stoichiometric 14.7 a/f mix which is ideal for passing emissions.
 

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Here Comes Trouble!
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The first test is OBD-II which is easily failed with any DTC lit up. The second test would be a sniffer test where trouble codes are irrelevant. Let's say you have a small exhaust leak. You will fail the OBD-II test because of the engine light. The sniffer test on the other hand is only concerned with the makeup of the exhaust leaving your tail pipes. As long as your cat converter is working you should easily pass the sniffer test.
A very good explanation. Rep points on the way.

:rep:
 
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