Q: Getting a CEL with a Code 1 "O2 Sensor" on a 1988 Honda Civic

I'm trying to chase down a CEL issue on my 1988 Honda Civic. I get Code 1 (ie. the light blinks 1 time on the ECU) so I know that it is related to the O2 sensor. So far I have done the following to try and isolate the problem: 1. Replaced the O2 sensor 2. Replaced the main relay 3. Adjusted the timing 4. Replaced the battery clamps 5. Checked to see if the ECU is bad - The CEL light comes on for 1-2 seconds when the key is in position 1 then goes off. I believe this indicates that the ECU is tested and good.

I'm wondering if it might be an exhaust leak out of the exhaust manifold gasket (or perhaps a cracked manifold?) but I don't hear any sounds around the manifold when it is running so I don't think that is the issue. I am also going to check to make sure that the engine is grounded correctly to make a circuit with the O2 sensor. Other than that I'm not sure what else to check.

My car has 211000 miles.
My car has a manual transmission.