Honda Repair: Check Engine light, h2so2, obd ii

Question
I have a '98 Civic and the check engine light came on.  I used a OBD II meter and the code is 0136...something regarding the oxygen sensor.  How likely is it the oxygen sensor?  Or is it the wiring harness going to the ECU?  My friend had a similiar problem and it was the wiring.  What do you suggest I do?

Answer
Larry,
As long as everything in your car is still "factory" (no aftermarket cat., intake, manifold, etc.) I would say you have two possible problems. One is a bad O2 sensor. Yes they do go bad sometimes. But the first thing you should check is your exhaust manifold. It may be cracked. I have seen a lot of these crack. Remove the upper heat shield on your exhaust manifold. (under the hood)Start the car and listen and look for a crack right between the 2 and 3 cylinders. This is where they crack. If you have a crack, that is the cause of the MIL light coming on. What is happening is the crack is sucking in unwanted air. This in turn sends a false "lean" condition to the PCM. The injector pulse duration is increased but cant compensate enough for the lean mixture. The PCM recognizes a problem and illumniates the MIL.
Basicly check for a cracked manifold. If its not cracked, replace the O2 sensor. Make sure you replace the right one. H2SO1 is the first one, H2So2 is the one in the cat. (Use your tester to make sure you know which one it is.
Good luck