The check engine light would come on after 20 minutes of running.
Well, to pass smog one of the things that has to happen is that the check engine light has to be off during the time of the smog check.
Smog check places will have an automatic fail if they see the check engine light on.
While waiting to get my own car smogged I would hear somethings from the smog check people.
They would explain to me that people try to bypass the system by disconnecting the battery and by doing so it resets and the check engine light goes off for a bit.
For awhile that would work, but to fix the problem, now if you bring in the a car with a recent reset it won't pass.
The smog check place now requires more history in the car computers or more miles driven after the reset.
So by requiring more miles after the reset will cause the check engine light to go back on like before the reset. Again if the check engine light is on it will cause an automatic fail.
The mechanics found that the oxygen sensor and it's wire connectors where faulty. So even with a new oxygen sensor their was still a disconnect as the wire connectors were corroded.
Mechanics were still stuck on why it still would not work till they noticed the corroded wire connectors to the oxygen sensor.
Well, they cleaned up the corroded part and now it works like it is supposed to.
You see the car was running too rich. Meaning that there was too much gas to air mixture in the engine causing black smoke to come out of the tail pipe.
Also when the car has a too rich of a gas to air mixture, the miles per gallon decreases from optimum.
Now the car is working right. There is no more black smoke and the check engine light won't come on even when the car is running for more than 20 minutes.
Mom is happy about that and so am I.
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Thank you, David.
Image is mine taken with my Moto G.