Carbon Monoxide Monitoring for Repair Shops, Upfitters and Parking Garages

Does an enclosed parking garage or repair shop need carbon monoxide detection?

You need either continuous full volume ventilation or CO and NO2 detectors controlling the ventilation. The code offers both, and the detector path is the one nearly everybody takes because the other one runs the fans all day.

Section 404.1 of the 2018 IMC states that mechanical ventilation systems for enclosed parking garages shall operate continuously or shall be automatically operated by means of carbon monoxide detectors applied in conjunction with nitrogen dioxide detectors, and that such detectors shall be listed in accordance with UL 2075. Section 404.5 applies the same ventilation requirements to motor vehicle repair garages.

The automatic path cycles the system between full on at not less than 0.75 cfm per square foot and standby at not less than 0.05 cfm per square foot. The difference between running at full volume all day and running at standby until a detector calls for air is most of the operating cost of the system.

So the honest way to describe this requirement is that detection is not literally mandatory, and the alternative is so expensive to operate that the point is academic.

Why nitrogen dioxide as well as carbon monoxide?

Because diesel exhaust and gasoline exhaust are different problems, and a CO detector alone will miss a garage full of diesel vehicles.

Gasoline engines produce carbon monoxide in quantity. Modern diesel engines, particularly those with aftertreatment, produce comparatively little CO but meaningful nitrogen dioxide. A fleet garage, a transit facility, a municipal yard or a truck upfitter running mostly diesel can have an NO2 problem in a space where the CO detectors never alarm.

That is why section 404.1 names both gases together rather than either one.

What concentration should the detectors alarm at?

The base code text does not set a ppm number. The setpoint comes from the UL 2075 listing and the manufacturer installation instructions, and your exposure obligation comes from the OSHA permissible exposure limit, which is a separate thing.

This is worth being careful about, because a great deal of published material states a code alarm setpoint for garage CO that the code does not contain. Section 404.1 requires detectors listed in accordance with UL 2075 and applied per their listing. It does not name a concentration.

The exposure limits are set elsewhere. Federally, 29 CFR 1910.1000 Table Z-1 gives carbon monoxide a permissible exposure limit of 50 ppm as an eight hour time weighted average. California is twice as strict: 8 CCR 5155 Table AC-1 sets 25 ppm as an eight hour time weighted average with a 200 ppm ceiling. If you are designing a system for a California shop, design to the California number.

Where should CO detectors be mounted in a garage?

At breathing height, distributed across the floor plate, per the detector listing. Not at the ceiling.

Carbon monoxide has a molecular weight of 28 against roughly 29 for air. For practical purposes it is neutrally buoyant and it goes where the air goes. It does not layer at the ceiling the way hydrogen does or pool at the floor the way carbon dioxide does.

That means the useful place to measure is where people breathe, and coverage is a question of spacing across the area rather than of height. The UL 2075 listing for the specific detector governs the spacing, and the mechanical engineer sizing the ventilation is the one who has to make the zones line up with the fans.

Is a shop that opens its bay doors exempt?

Not automatically, and this is a common and expensive assumption.

The code provision applies to enclosed parking garages and to repair garages by reference. Whether a particular building counts as enclosed is a determination the code official makes from the building geometry and openings, not something an operator settles by propping a door.

Separately, and regardless of how the mechanical code classifies the building, the employer still owes its employees an atmosphere below the permissible exposure limit. An open door on a still day moves very little air. If your people run engines indoors, the question of whether they are over 25 or 50 ppm is answered by measurement, not by the door position.

Primary sources

M Squared Safety Solutions, Inc., 981 Calle Negocio Suite 200, San Clemente, CA 92673, 949-954-6581. Every code and regulatory statement on this page was read against the cited source on 18 September 2026. Model code section numbers are cited to the 2018 International Fire Code and International Mechanical Code. Jurisdictions adopt and amend these codes separately, so confirm the edition in force where the work is. This page describes what the regulations say. Whether a given provision is satisfied at a particular facility is a determination for that employer and the code official.