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I'm not an expert but.....
If your upstream sensors aren't connected... the CEL will be on and the engine will just run really rich so you will get terrible gas mileage...
If your downstream sensors aren't connected...you will get a CEL with codes related to the catalytic converter and/or downstream o2 sensors
either way you will get a CEL...but the ECM has protections built in to prevent from running a STOCK engine too lean
AND if you mean unscrewed from the exhaust pipe...you will have an exhaust leak pointed towards wherever the bung is = not good for whatever is in the way.....
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