Altima multiple Emission failures

This 2000 Nissan Altima came in for Emission test.
It failed on CO. CO reading was 3 times of its limit, but everything
else passed. High CO means too much incomplete combustion is taking
place inside engine. We analysed as usual from the basics. When we scanned the engine
data, we found engine temperature read low
than actual engine temperature. (1) We replaced the ECT sensor, and
retested.

The CO came down to 1/5 of original reading, so it did corrected the
rich running condition. But NO went up almost 2.5 times higher than original testing.  The
engine was running too rich(=CO was too high), and this caused the lower
NO emission. Now we narrowed the diagnostic to EGR problem. We found that
back pressure rubber tubing was damaged and broken. (2) We replaced the special heat
resistance hose, and retested.

It failed again. By replacing this hose, NO increased slightly higher.
We further found that back pressure metal tubings were badly clogged. We
had to remove 2 of them. (3) We cleaned off accumulated carbon, and drill
out the clogged hole on both pipes. We blew compressed air into tubes
and made sure them from any more cloggings. Retest for the last time,
and finally the emission was low enough to pass the test. I believe
that if we cleaned the combustion chamber with de carbonizer, the NO emission
would be further reduce.

This car had multiple problems.  We were happy to solve this pollutant
vehicle and protect clean air.  The customer will enjoy better fuel
economy for sure.