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It was indeed the catalytic converter (the EX does not have a pre-cat). After replacing it, the downstream sensor graph spends most of its time above 0.5 V, as it should if the converter is working. Update: I started getting this error code again, a little over a year after replacing the cat. I also finally found a guide on how to read these graphs, from ...

I've diagnosed this condition many times with a vehicle-specific scan tool, and I don't think it can be done without this or an oscilloscope. O2 sensors work in the range of less than 1 volt (.2v to .7v) In particular, the rear O2 sensor should stay at a fairly constant .5v if the cat is working right. Wild swings to the extremes indicate the cat isn't ...

Since you have new plugs and wires you may have a bad coil. Of course one of the new parts may be bad, not unheard of but unlikely. If the fuel injected into the cylinder isn't burned it passes into the exhaust system where the O2 sensor sees a rich condition that is outside the normal parameters and sets a fault code. You need to see if that cylinder is ...

A common problem in civics of that year. It's not the main catalyst that goes bad, it's the pre-cat (the one hanging right off the exhaust manifold). This one is for cold start and once your car is warmed up, the lions share of the work is done by your main catalyst. Take it in to the dealer to have your precat checked. Many cars have a 10 year warranty ...

Symptoms for a bad EGR valve and a bad MAF sensor aren't too different, it is a fair possibility that the MAF needs to be cleaned (easy to do). Just take the MAF sensor out, clean the electrical connection and spray through the sensor with some MAF cleaner or (though it's harsher it does work) Carb cleaner. Inspect the element, it should be a small silver ...

Speaking only in generalities (not being familiar with Fords of any kind), EGR systems fall victim to damaged vacuum lines occasionally, so check those. They can also clog up with carbon enough to fail to operate properly. Neither would normally happen in so few miles, but if the load was heavy enough, I suppose the enrichment program might lead to the ...