Arrived to furnace maintenance. Upon arrival I had the thermostat call for heat and made my way to the indoor unit. Here the inducer motor, igniter, and flame sensor (cleaned) performed within spec. The blower was drawing correct amperage as well. I performed a combustion analysis with this performing within spec. I then performed a gas leak search finding none present. I did notice the water heater did not have an expansion tank on it which is now required due to code. I spoke about this and the age of the water heater with the customer. System running upon departure.