I have a 91 honda civic. 2 weeks ago, the CEL went on. It was a single red flash on the ECU. which is the O2 sensor. I replaced the sensor and cleared the Code by removing the battery overnight.
Today on my way home the CEL went on again, same code.
What could be causing this?
it leaks some oil from the valve cover, and I recently replaced the head gasket. When I start the car up it smells rich.

 

O2 Sensor is not bad, you have a sensor that is doing it's job. You will need to fix your rich running car.

 

Where is a good place to start trouble shooting? Could the dashpot tandem control valve cause it to run rich?

 

When you changed your head gasket, you may have put the timing belt back on one tooth off. Check your cam timing then you may want to check your ignition timing.

 

Today I chck the voltage to my tandem valve control soleniod. I'm missing the 12 volts. Could that mak the O2 go out. Also the build-up on the O2 was white.

 

I think I am having the exact same problem. Except one time I drove through rather large puddle. (Unintentionally) Car acted up for a couple hours. Parked it. Next day worked fine...... weird.

 

Wouldn't it run really bad if I was off one tooth? This car drives well, sounds great. I pulled the newer O2 sensor yestrday. it looked like new. No build up on it. I have ot had the CEL light on since I replaced it. But, this was the same with the last O2 sensor.

 

You could have a problem in the O2 sensor wiring or it could be running rich as mentioned.

Test your fuel pressure regulator, make sure its ok.
Look towards the bottom for testing: http://www.hondacivicforum.com/forum...6-issue-61252/

Also test your O2 sensor curcuit: http://www.justanswer.com/honda/11xt...otor-into.html