Carbon monoxide is the diagnosis an HVAC tech can't make by feel — it's colorless, odorless, and the difference between 30 ppm and 300 ppm is the difference between a note on the invoice and an evacuation.
The Klein ET110 puts the number on a backlit LCD: CO concentration from 0 to 1000 ppm at 1 ppm resolution, with a replaceable electrochemical sensor rated for a 5-year life and a T90 response under 60 seconds. Accuracy runs +/-5% (or 5 ppm) in the 59-77°F midrange, and MAX/AVG functions cycle through maximum, minimum, and average readings for documenting a call. Ambient temperature displays in Fahrenheit or Celsius.

The alarm structure does the safety thinking with you.
A low-level alarm trips at 35 ppm (the OSHA/NIOSH 8-hour exposure benchmark) and a high-level alarm at 200 ppm, both audible and visual with a warning indicator light — and the short-term exposure limit (STEL) alarm (200 ppm over 15 minutes) adds protection against the accumulating doses a simple threshold misses. Auto power-off after 20 minutes of inactivity conserves the four included AAA batteries.

Two honest cautions, straight from Klein: this meter is not intended for use as personal protective equipment, and it is not insulated.
It's a diagnostic instrument for finding, measuring, and documenting CO conditions at furnaces, flues, boiler rooms, and gas appliances — not a substitute for a personal monitor in confined-space work. For HVAC pros, inspectors, and property techs, it's the fast answer to "is this appliance making CO, and how much." Carrying pouch and batteries included; 3.3 ft drop protection.

• Measures CO from 0 to 1000 ppm at 1 ppm resolution; temperature in °F/°C
• Low alarm at 35 ppm; high alarm at 200 ppm; STEL alarm (200 ppm / 15 min)
• Audible and visual alarms with warning indicator light
• Replaceable electrochemical sensor, 5-year life expectancy; T90 response < 60 seconds
• MAX / AVG / MIN reading functions; backlit LCD
• Auto power-off after 20 minutes conserves batteries
• Includes carrying pouch and 4x AAA batteries
• Not PPE; not insulated

• Model: Klein Tools ET110
• CO Range: 0 - 1000 ppm, 1 ppm resolution
• Accuracy: +/-5% or 5 ppm (59-77°F); +/-10% or 10 ppm elsewhere in range
• Response: T90 < 60 seconds
• Sensor: Electrochemical, 5-year life
• Alarms: 35 ppm low, 200 ppm high, STEL (200 ppm / 15 min)
• Weight: 6.87 oz with batteries; drop protection 3.3 ft (1 m)
• Power: 4x AAA (included)
• UPC: 092644690655