My 5 speed 1990 honda civic si runs very well but just recently the check engine lights comes on and off. I did a full tune up already (oil, filter, plugs, wires, pcv, fuel filter, o2 sensor). The thing is it only comes on after i hit 50 mph. I have tested my suspicions, and this happens every time. What could be the problem?

the mechanic i brought it too checked the ecu for flashes and replaced an o2 sensor because of a code..but now its on again, and its pretty hard to get him to check the ecu when the car is running, because thats the only time the check engine light is on and i cant open my door when my keys are in the ignition. Anyone had the same problem?

 

I say it's your ecu. change it out to another one to check.

 

sounds like an o2 sensor. Do you know how to read your ecu?