p0304,p0400,p0420 cause related?

[tedt]

I recently bought a '96 Legacy GT with 195,000 miles
from a friend. It has a history of the dealer changing
the EGR valve. It is now showing the codes P0304 (
cylinder #4 misfire) P0400 ( EGR flow malfunction) P0420
(catalyst system efficiency below threshold). It runs
good but mpg and pick-up seem a bit low. My question
is if these could be related and what problem should
I tackle first.

Thanks,
Ted

 

[Carl 1 Lucky Texan]

At this mileage you're probably due for the 2nd possibly 3rd front (at
least) O2 sensor. Possibly new cat converter - some mechs have a
pressure test for the cat using the O2 bung hole.

Not clear in your post, but if the other codes come back immediately
after clearing them they could be real second and 3rd issues, but I have
read about bad cats causing misfires/pinging/loss of power etc.

If you want a lot of info and help on tackling these problems yourself
check the forums at www.usmb.net

Carl
1 Lucky Texan

 

[TG]

The "low catalyst efficiency" is usually caused by air entering the exhaust
system where the headers bolt to the head (exhaust manifold). Buy new
gaskets, studs, and nuts and watch the torque. You might be able to get away
with just gaskets but chase the threads on the studs and nuts if you do it
that way otherwise the torque readings will be insufficient to seal things
up and additional torque will break a stud. TG

 

[GTT]

The "low catalyst efficiency" is usually caused by air entering the exhaust
system where the headers bolt to the head (exhaust manifold). Buy new
gaskets, studs, and nuts and watch the torque. You might be able to get away
with just gaskets but chase the threads on the studs and nuts if you do it
that way otherwise the torque readings will be insufficient to seal things
up and additional torque will break a stud. TG

err, I see this is a fairly old post, but could you explain "chase the
threads ..."
for this old feller?

Thanks,