What Causes High Carbon Monoxide (CO)?
Below are common faults which are likely to produce high Carbon Monoxides (CO) and Rich Fuel Mixtures. Carbon Monoxide is a by-product of incomplete combustion. Carbon Monoxide exceeding maximum limits, can be due to a number of emission failures ranging from inadequate air intake to defective engine computer sensors. This condition is referred to as a "Rich Fuel Conditon".
Why causes a rich fuel condition?
1. Dirty Air Filter - The number one overlooked emissions component, yes, "emissions" component is the engine air filter. A dirty air filter will absolutely restrict air flow, thus disturbing the proper 14.7:1 air/fuel ratio required for optimum fuel combustion.
We recommend replacing the air filter at the manufacturer's required intervals; usually every 15,000 miles, or at least before your vehicle's smog check.
2. Faulty Oxygen Sensor (O2 Sensor) The Oxygen Sensor is responsibly for delivering information to the ECU (engine control unit) or ECM (engine control module) relating to the oxygen content in the exhaust stream after it has left the combustion chambers.
The engine control computer will determine how much fuel to inject into the combustion chambers based on this data. The more oxygen in the stream, the more fuel the computer will deliver, and visa-versa. A defective O2 sensor will cause increased carbon monoxide emissions. More about oxygen sensors.
3. Defective Manifold Absolute Pressure (MAP) Sensor - The MAP sensor determines the level of vacuum created during an engine's intake stroke, and sends this information to the ECU. During low vacuum the MAP sensor assumes the engine's throttle is in some degree open, meaning you've stepped on the pedal. It relays this information to the ECU. The ECU, in turn, sends commands to the fuel injectors, or carburetor, to increase fuel delivery.
A defective MAP sensor will not report the correct information to the ECU, thus disturbing air/fuel ratio. Usually when the ECU senses a defective MAP sensor it will learn to ignore its data, and rely on preset values, and other sensors such as the Throttle Position Sensor (TPS), and Engine Coolant Temperature (ECT) Sensor; Fuel delivery will not be as accurate and high CO may result.