by Lance Wright


The vehicle has failed an emissions test or the exhaust emissions are too high as measured by an emissions analyzer.

There are numerous components that if not working properly, can cause high emissions or an emissions test failure. You should read each of the components description and operation in the emission control system section of the manufacturer’s manual covering your vehicle in order to gain a working knowledge of each system. These systems are designed to reduce the emissions (as set by California emissions standards). If any of these emissions components fail, emissions can increase. Along with these systems, other components such as spark plugs, plug wires, and filters can also increase emissions if defective. Emissions or smog is a generic term used to describe certain harmful gases in the exhaust. Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOX) are usually measured in an emissions test. The state and federal government place certain limits on each of these harmful gases emitted. Carbon Dioxide (CO2) is also measured, but is not considered a harmful gas. When the measured gases from the vehicle exceed the state or federal limits, the vehicle fails the emissions test.

Diagnosis of high emissions can be an involved process that usually requires special tools and measuring equipment. However, armed with a test report indicating what gases are high can help you determine what systems may not be functional. If the vehicle has failed as a result of high CO and HC, the CO failure should be diagnosed first. If the “Service Engine Soon” or “Check Engine” light is on it should be diagnosed prior to any emissions diagnosis. CO Failure is the result of an excessively rich air fuel mixture. The following is a list of the most common causes:
– Defective Oxygen Sensor
– Inoperative Air Injection System (if equipped)
– Leaking or defective Fuel Injectors
– Restricted air filter (especially if high CO is present only at high RPM)
– Vacuum leaks resulting in improper MAP sensor operation
– Defective air mass or air flow sensor
– Malfunctioning fuel evaporation system or purge valve
– Defective Catalytic Converter
– Defective thermostat (cooling system), thermostat stuck open
– Oil contaminated with fuel, excessive miles between oil changes

HC Failure is the result of incomplete combustion. Any item that causes incomplete combustion can result in high HC. The most common causes are:
– Worn spark plugs
– Defective spark plug wires
– Worn distributor cap and/or rotor
– Improper ignition timing, usually over advanced
– Vacuum leaks
– Engine mechanical failure, low compression, worn valves, excessive oil consumption

NOX Failure is the result of combustion temperatures that are too high. The most common causes are:
– Inoperative Exhaust Gas Recirculation System (EGR)
– Cooling system malfunction, engine running too hot (restricted radiator, defective thermostat, etc.)
– Over advanced ignition timing
– Excessively lean air fuel mixture (defective oxygen sensor, MAP sensor or Air Mass Sensor)

If the engine is due for a tune up it is wise to perform preventative maintenance prior to spending too much time diagnosing the emissions problem. Proper vehicle maintenance can go a long way in keeping a vehicle’s emissions within specifications. A complete tune-up should consist of: spark plugs, ditributor cap and rotor (if equipped), spark plug wires, air filter, fuel filter, and oil change. Although performing this service is no gaurantee of a passed emissions test, it certainly increases the likelyhood of success. If the vehicle has over 60,000 miles on it and has failed for the reason of high CO, installation of a new oxygen sensor might be a prudent investment especially if the vehicle is only equipped with one sensor.