The amount of carbon monoxide (CO) a car produces varies significantly based on its age, technology, and maintenance, but a well-maintained modern car emits very little. On average, a gasoline car emits about 0.05 grams of CO per mile under normal driving conditions. However, a poorly tuned or older vehicle can produce over 10 times that amount. The key factor is the efficiency of the catalytic converter, which oxidizes over 99% of CO into less harmful carbon dioxide (CO2) when functioning correctly.
The following table illustrates how different factors influence a vehicle's CO emissions, using data from the U.S. Environmental Protection Agency (EPA) and industry studies.
| Factor | Low CO Emission Scenario (grams/mile) | High CO Emission Scenario (grams/mile) | Notes |
|---|
| Vehicle Age | 0.02 (2020+ model, modern emissions controls) | 1.5+ (Pre-1980 model, no catalytic converter) | Stricter EPA Tier 3 standards have drastically reduced allowable emissions. |
| Engine Tuning | 0.05 (Properly tuned engine, correct air-fuel ratio) | 3.0+ (Faulty oxygen sensor, rich fuel mixture) | A "rich" mixture means too much fuel, leading to incomplete combustion and high CO. |
| Catalytic Converter | < 0.01 (New, functioning optimally) | 5.0+ (Failed or missing converter) | This is the most critical emissions control device for CO reduction. |
| Cold Start | 0.1 (Engine at operating temperature) | 2.0+ (First few minutes of operation) | Emissions controls are less effective until the engine and catalytic converter warm up. |
| Driving Conditions | 0.03 (Steady highway cruising) | 0.3 (Aggressive city driving, frequent stops) | Hard acceleration demands more fuel, which can temporarily create a richer mixture. |
The primary source of carbon monoxide in a car's exhaust is incomplete combustion of gasoline. When there isn't enough oxygen to fully convert the fuel's carbon into CO2, CO is formed. This is why a malfunctioning engine—often with a faulty oxygen sensor, spark plug, or fuel injector—is a major polluter.
The most effective ways to minimize your car's CO output are straightforward: adhere to the manufacturer's recommended maintenance schedule, promptly address any check engine lights, and avoid unnecessary idling. A properly maintained modern vehicle contributes minimally to ambient CO levels, which is a significant public health achievement since the 1970s.
