Sorry but it does not allow for a CO detection system as the sole method.
2009 IMC
502.14 Motor vehicle operation.
In areas where motor vehicles operate, mechanical ventilation shall be provided in accordance with Section 403. Additionally, areas in which stationary motor vehicles are operated shall be provided with a source capture system that connects directly to the motor vehicle exhaust systems.
Exceptions:
1. This section shall not apply where the motor vehicles being operated or repaired are electrically powered.
2. This section shall not apply to one- and two-family dwellings.
3. This section shall not apply to motor vehicle service areas where engines are operated inside the building only for the duration necessary to move the motor vehicles in and out of the building.
Depending on the type of repairs this might be the only exception applicable. I would only allow in a tire, transmission, body and exhaust shop to name a few.
[F] 502.15 Repair garages.
Where Class I liquids or LP-gas are stored or used within a building having a basement or pit wherein flammable vapors could accumulate, the basement or pit shall be provided with ventilation designed to prevent the accumulation of flammable vapors therein.
[F] 502.16 Repair garages for natural gas- and hydrogen-fueled vehicles.
Repair garages used for the repair of natural gas- or hydrogen-fueled vehicles shall be provided with an approved mechanical ventilation system. The mechanical ventilation system shall be in accordance with Sections 502.16.1 and 502.16.2.
Exception: Where approved by the code official, natural ventilation shall be permitted in lieu of mechanical ventilation.
[F] 502.16.1 Design.
Indoor locations shall be ventilated utilizing air supply inlets and exhaust outlets arranged to provide uniform air movement to the extent practical. Inlets shall be uniformly arranged on exterior walls near floor level. Outlets shall be located at the high point of the room in exterior walls or the roof.
Ventilation shall be by a continuous mechanical ventilation system or by a mechanical ventilation system activated by a continuously monitoring natural gas detection system, or for hydrogen, a continuously monitoring flammable gas detection system, each activating at a gas concentration of 25 percent of the lower flammable limit (LFL). In all cases, the system shall shut down the fueling system in the event of failure of the ventilation system.
The ventilation rate shall be at least 1 cubic foot per minute per 12 cubic feet [0.00138 m3/(s • m3)] of room volume.
[F] 502.16.2 Operation.
The mechanical ventilation system shall operate continuously.
Exceptions:
1. Mechanical ventilation systems that are interlocked with a gas detection system designed in accordance with the International Fire Code.
2. Mechanical ventilation systems in garages that are used only for the repair of vehicles fueled by liquid fuels or odorized gases, such as CNG, where the ventilation system is electrically interlocked with the lighting circuit.
