I just took my 98 Mazda 626 for it's inspection and they said the carbon monoxide is slightly high. The test is done using a tail pipe gas analyzer while holding the RPMs between 2500 - 2800 for thirty seconds.
From what I've read, this is normally an indication that something is causing the engine to run rich:
Carbon monoxide (CO), is incompletely burned fuel or to be more precise are hydrocarbon molecules that split apart but don't burn in the combustion cycle. High (CO) is the result of one problem, a rich air/fuel mixture but may have several causes.
My CO reading was 0.6% by volume, with the manufacturer's limit being 0.3%.
The was no problem with NOx emissions.
However, during this test they also take a Lambda reading. My reading was slightly lean at 1.021, with the limit being 1.03. This reading seems reasonable as my fuel trims are close to zero. According what I've read in Engine Management: Advanced Tuning, this slightly lean lambda is a very reasonable target for steady state cruising conditions:
For best economy we swing to the other side of the stoichiometric balance with a target of about Lambda = 1.05 ( about 15.5:1 A/F ). ... To achieve this, we mix just enough fuel that, if burned completely, makes enough power to maintain current conditions with enough excess oxygen to ensure that no fuel molecules go unused.
So based on the objective readings of the gas analyzer I'm running both rich and lean at the same time!
Now, for anyone who's been following the adventures of my 626 project, you'll know that I've got a brand new O2 sensor, and that I had been having problems with hesitation, lack of power and rough idle. These problems were greatly improved, but not eliminated, by replacing my standard spark plugs with fine tip low voltage NGK g-power plugs.
Now I think these things are related. Having both incompletely burnt fuel and excess oxygen says to me that I'm getting poor / weak ignition of the A/F mixture, causing a slow, incomplete burn.
I've used a spark tester at night set to a 30kv gap which produced a weak thin looking orange spark, when based on folk wisdom I should be getting a thick bluish / white spark. I've tested resistance on my wires and they're all in spec.
My theory is that I've got a weak ignition coil. The Mazda WSM calls for three tests of the coil pack. The primary and secondary resistances are in spec, although it takes a second or two for the reading on the primary coil to settle. It's starts high and then slowly settles to the correct value of between 0.45 to 0.55 ohms. I was unable to do the third test, which is the "Insulation Resistance of Case" test calling to measure resistance between the ground terminal and the case with a "500 v Mega tester". I was also unable to do the conductance test suggested by another user.
The I've checked pretty much everything else under the sun, and this is the only thing that's making sense to me, so I ordered one which should be here in about a week.
I'd like to hear what people think about my theory.
Postscript April 8th, 2016
So I went in to do the test again today, and it passed emissions this time. This time my lambda was 1.009 and the CO was 0.1%. I'm not completely sure why, so I'll just list everything that was different.
First off, I had recently installed a fuel pressure gauge and had to remove the air intake hose. I forgot to tighten down the band where the hose meets the throttle body, so I tightened it down. It might be that a tiny bit of air was getting in there.
The first time I did the test I was idling in line for 20 minutes and they read engine temp as 126 C*. This time I went straight in, it was 96 C*, and they had me rev the engine hard three times before starting the test, and also had me rev up to 3000, although their equipment showed 2770.
Post-Post Script March 3rd, 2017
Thought I'd follow up again after this years emissions test.
Well, it failed emissions again when I went for the test last week, but just barely. Idling at 670 rpm according to their equipment, CO was at 0.51% with a maximum allowed of 0.50%, so it just barely failed there. Held at 2770 rpm and measuring 80*C both according to their equipment for 30 seconds the lambda read 1.017 which was fine, but the CO read 0.46% with a max allowed of 0.3%.
After reviewing what @FredWilson wrote here last year, I figured that since 98% of my driving is low speed, low rpm in the city, maybe taking the car for a high rpm drive on the highway for 15 minutes might burn off any carbon or sulfur that was interfering with the cat's performance. So I did that and went for a retest ( a week after the first test ). This time it read 0.04% idling at 670 rpm. At 2770 rpm and 91*C according to their equipment they read a 1.014 lamba and 0.18% CO, so it passed this time.
Some side notes; on the second test I was again very close to 3000 rpm according to my tac, which makes me think there is a problem with their equipment maxing out at 2770 rpm. Also, the first time I did the test the car was idling pretty rough, while, for no apparent reason the idle was much smoother the second time I did the test. Not sure if this was a contributing factor, or just coincidence.