A change in the peak CO reading on light off, for example, will alert you that a component may be beginning to fail - long before it actually becomes a no heat call. As previously mentioned, all burners are designed and engineered to operate with specific ranges of excess combustion air. To review a 'generic' list of acceptable oxygen or carbon dioxide readings refer to the Combustion Zone in the Training Room or preferably check with the manufacturer for their specifications. As you are aware, this is a very competitive business. Properly used, this equipment will help maintain your existing customer base and generate additional sales.
Satisfied customers pass the word around.
Obviously, your 'first impression' means a lot when dealing with a potential customer. That alone will make the sale in many cases. It demonstrates to the customer that you have the ability to insure safe, reliable and efficient heating system operation. It also provides the homeowner some valuable information with regards to the most cost effective furnace replacement, for example. In other cases, combustion tests may identify the reason replacement is being considered in the first place and help troubleshoot and properly service the system or verify that replacement is indeed necessary.