I pose a question to trained service and repair technicians:
What CO meter would you recommend as an affordable and dependable shop tool to use as reference and/or verification of fuel/air ratio in CIS K and KE-Jetronic applications?

A tool that is available to purchase today

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I do not 'believe'. I have no use or time for this distraction. A'priori is antiquated furniture that clutters the mind and deteriorates society. Instead I investigate, research, scrutinize then choose from data what best serves as a suitable answer until a new idea or material is presented. Then I start the process again.