First, can a dealer learn anything more about a problem than I can by the CEL?

The CEL on my 98 Civic HX indicates misfire in cylinders 1, 2, 3, and 4, and that there is a problem with the O2 sensor - I think I remember the code was 71. When the problem occurs the engine runs rough.

I would replace the O2 sensor except I am not sure that is the problem.

The operational problem - when it occurs - is that the engine runs rough. Through a couple of months of observation, it appears that 2 conditions must occur for the problem to manifest itself.

1. The engine must be warm - it never occurs at first start-up and for several minutes after.

AND --

The air temperature must be at least 80 F. I don't believe it occurs at outside air temperatures less than 80 F.

It seems to be an ON/OFF thing except that it does seem to get a little worse the longer the trip is although that really could be a function of outside air temperature increasing.

The mpg has not seemed to change.

I cannot think of a sensor problem that would be dependent upon air temperature.

 

If it has the cat. converter on the front of the engine like a DX (I don't remember if the HX does or not), check the manifold for cracks. The integral manifold/converter is famous for cracking. A crack will let air in and fool the O2 sensor, causing an improper fuel mixture. This only starts after the engine warms up because the sensor is ignored (open loop mode) while the engine is warming up.