I don't know if an automotive emissions analyzer would cover all the gases of interest, but it might be a start. The instrument makers Bacharach and Beckmann might offer some good choices, maybe even off-the-shelf what you need. Web searching "flue gas analyzers" could turn up some leads also. I haven't run a boiler in some years, but some of the boiler folks here can probably suggest.
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It's best to install the sensor in the outlet flue before the tip of the exhaust stack.
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