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.

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84 300D, 82 Volvo 244Gl Diesel