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The oxygen sensor on a 1999 Honda Civic is located on top of the catalytic converter. The catalytic converter is under the car.
The converter will cost from $275 to $325 depending on where you buy it. Labor to remove the old one and install a new one will be around $100.
The manifold is considered before the catalytic converter.
There should be one in the header and one more in the catalytic converter.
The O2 sensor on a Honda Civic is found on the exhaust system. It will be mounted near the catalytic converter. Simply unscrew the sensor from the exhaust pipe and unplug it from the wire harness. Replace with a new unit.
No, it is illegal to remove a converter and besides the engine will not run right with the converter removed.
Very carefully
There is one before the catalytic converter where the 4 runners meet on the exhaust manifold. And the second one is after the catalytic converter.
It is the one after the catalytic converter?
There is one and it is before the catalytic converter.
The primary oxygen sensor is located before the catalytic on most Hondas depending on the year. If your Honda has the catalytic converter integrated to the exhaust manifold then you should find it on the manifold just before the catalytic converter.
You either removed the catalytic converter or you are running too rich.