Admittedly one does not want to replace things willy-nilly
at this point. And you say the Code 12 continues. (I also
confirmed Code 12 is symptomatic of a "problem" with the EGR
system.) Using the manuals at the sites below, I would be
checking the EGR's electrical connections first. Yes, it's
new, so this suggests that, if it's not working, it's more
likely that the connections were not made correctly.
For troubleshooting codes yada for your Prelude:
Your Prelude's factory service manual:
Next I would do what Motsco said. One must take seriously
purging air from the coolant system. If any repairs you had
done recently involved the coolant system in any way, I'd do
a purge, per the manual. It may take 50 minutes or so for
your car to warm up during the purge, so be patient. The
level appearing to be fine is not enough to indicate air is
If this Prelude is on its original, OEM oxygen sensor,
nonetheless I would consider replacing it. Your symptoms do
suggest a failing O2 sensor. Particularly the symptom that
it starts going amiss after warmup. If it is the original O2
sensor, then it is in fact quite old. A code will not
necessarily be thrown for its malfunction. You can get an
OEM O2 sensor from www.automedicsupply.com for under $40
total (= about $32 + shipping). In 2004, I used this online
store for a new O2 sensor for my 91 Civic (the Denso one).
Great service. No problems with the new O2 sensor.
Based only on reading here and a bit of googling, I'd also
investigate whether the catalytic converter was partly
plugged. Google on same for more info. Of course, then the
big question is how it got plugged. Related to the EGR
Keep checking back. Others with more expertise will post.
Original owner and now sole mantainer, 1991 Civic