Failed Smog/Emission Test high Carbon Monoxide too high
 

If your Carbon Monoxide is too high you Hydrocarbons are going to also be too high because the basic cause is too much fuel or fuel for some reason not burning as it should.
Failed test CO% (average = the average emissions for passing vehicles in genera.)
Tested 15 mph, Max=0.48, Average= 0.08, measured= 1.72
Tested 25mph, Max= 0.62, Average= 0.09, measured= 2.13

Passing Test CO%
Tested 15 mph, Max=0.48, Average= 0.08, measured= 0.01
Tested 25mph, Max= 0.62, Average= 0.09, measured= 0.00
Note I did not put them down but the Hydrocarbon/HC levels also went back to passing. I was told the passing emission test was like it was a new vehicle.

You can go on the internet and get a list of possible causes as individual cuses or multiple causes.

What this thread is really about is that without something to test your exhaust gas how are you going to tell if something you did fixed the problem? Where I go you get one free test after you failed. After that you pay again and start over.

First step is to look to see of a computer code was generated. My 92 Chevy Astro Van is an ODBI so that means there is not much monitored compared to the newer vehicles.

The check engine light has been going on intermittently for like 8 years and generates a code #22 which is low output voltage at the Throttle Positon Sensor. That code is the only Code present and was there the last 3 times I passed the Smog/Emission Test so I did not expect that to be the issue.
However, when test on the TPS as in the manual it passes???

Back to how you tell if what you did corrected the problem?

I bought a Carbon Monoxide Meter/Tester (not the one like you use for Furnaces). It was about $200 for a new one. (I actually have one from the 1970s but was not sure it worked anymore so I bought a new one.)
They used to be used in the old days go adjust Carburators on the vehicle. In this case I needed to know if what I did reduced the CO.

Another name for them is air/fuel ratio gauges/meters/testers.

So as I said I had tested the TPS and found nothing obvious. Thinking I might have a miss I pulled all of the Spark Plugs and Spark Plug Wires and Ohmed out the Plug Wires and found nothing obvious like discoloration on the spark plugs.
I installed new Plugs and reused the Wires and tested with the CO Tester and no change. Still too much CO

There was no computer code saying there was an issue with the O2 sensor but I got a Bosch one cheap on eBay and replaced that and re-tested with the Co Tester and that fixed the issue.
Went down had the emission test done again and passed.

I am going to put a picture of the one I bought on ebay from a US Seller. Lots of the same maker used and new ones on sale but from England (where they are made) but I wanted to get it faster and a new one.

The first picture is the green Gunson Gasmaster Part No G4125 digital; mine is entirly black.
The yellow one is a Gunson's Gastester MK2 with an analog meter. That is the one I bought back in the 1970s However the new Gunson actually bought out the old Gunson's who made the MK2.

So back to the delema. I spent $200 on the CO Meter and $160 on an Actron Engine Analyzer which is not much use on ODBI however it also does OBII of the newer US cars.

If I had paid someone I would have had to pay again if there was an issue with my other 2 gasoline vehicles. I hope I will be able to use the equipment later.

I am also hoping to get my old CO tester to function properly and I can sell it.

The newer digital one is better for computer controlled systems because it reads lower then .00 it goes into negative.

The old one with the analog meter is still OK for carbureted cars.

I had another vehicle that needed the smog test and I tested it also with the CO tester and it showed no issues and it passed the first time.

Warning I did not spellcheck.

Note that there is extremely vintage air/fuel ration testers/meters/gauges on ebay and others made at a later date. However, most of the used ones the seller does not know how to test them and/or they are incomplete. Sellers often have never seen the completed unit and don't now if there is pieces missing or not. Some have no instructions.

So I did not want to take a chance with a used especially untested one.

You can go (automotive use) 2 steps above that with a made in the PRC new 2 gas analyzer about $1200 or go for a 5 gass analyzer for $2400. Those have a probe that goes into the Exhaust Pipe.
The 5 gas would be great to check your Car with but since it comes direct from the PRC what would happen if you had an issue with it. The same or similar 5 gas analyzers sold here and likely still made in the PRC are $5000.

Then there is fuel ration gauges or analyzers that you need to drill and weld on a bung to fit an O2 sensor into your exaust pipe. A lot of them have a regular gauge for in your Cab but the generally show the mixture like 14:1 and so on. Not the carbonmonoxide. However there is different gauges. they are in the $300-$350 range.

Then there is the Analyzers with a similar setup with the O2 sensor in the Exhaust but is a hand held device with a screen and with some added stuff can be connected to a Laptop to do other stuff. $450 and way up.
From the little I read about them people are using them while the vehicle is running to test their fuel delivery over a whole range of operation. Hence the computer connection and software.

Then there is a DIY type with an O2 sensor that you simply read the Voltage from the O2 Sensor and estimate the Fuel Mixture which intern give you some idea of the CO. But you need to drill the exhaust and weld in a Bung for the O2 Sensor.

Peaking & Tweaking
 

Thanx for this .

I used to do pre smog tests way back when (you're in Cali., remember the wretched hamilton test centers ?) and back then N02 wasn't on anyone's radar so we used the two gas sniffer and would run the Co down until the HC began to rise, then tweak it until we found the sweet spot .

I was fiddling with an old Honda Motocycle to - day and wishing I had a two gas analyzer now .

I had the excessive NO2 issue on one my Cars and fortunately installing a new CAT fixed that issue. You don't have to buy the direct fit ones but do have to get in my case a California approved one and what ever adapters you are going to need to fit it in.
And, they are cheaper on the internet.