There are three basic sections of an OBD II Catalytic Converter.
- The first section is called the reduction section; it reduces NOx down to oxygen and nitrogen.
- The Cerium section of the Catalytic Converter stores oxygen, where it is used in the oxidizing section, converting CO and raw HCs into CO2 and 02. This section is designed to maintain a predetermined threshold of oxygen so if the Catalytic Converter exhaust oxygen content gets too low, the Cerium releases it. If it gets too high, it stores it, thereby maintaining an ideal or Stoichiometric ratio of exhaust gases. (Stoichiometric is an ideal air to fuel ratio that maintains the cleanest balance of emissions "after burning" in the Catalytic Converter. Numerically, it is 14.7 parts air to 1 part fuel.)
- The Catalytic Converter Monitor uses the voltage and response times of the front oxygen sensors and rear oxygen sensors to determine the "scrubbing" ability of the CAT. The monitor watches the behavior of the front oxygen sensor in relation to the rear oxygen sensor on each bank or line of engine cylinders. The front oxygen sensor is usually near the exhaust port, in or near the exhaust manifold, but before the inlet of the CAT. The rear oxygen sensor is near or on the rear outlet of the CAT. The CAT Monitor wants to see a change in behavior from the front oxygen sensor versus the rear oxygen sensor. Depending on the change in electrical signals from the front to rear sensors, as the exhaust gases are converted from flowing through the CAT, the Catalytic Monitor determines that the converter is effective and within specified tolerance, as set by the manufacturer and in accordance with EPA guidelines.
The Catalytic Monitor is mostly run at highway speeds, usually at 55–60 MPH for two to four minutes. On some LEV or ULEV (low emission or ultra low emission) vehicles, the CAT Monitor is run at several speeds, including city speeds, idle, and even hard acceleration. This is why some vehicles need very sophisticated Air Fuel Ratio-type oxygen sensors that can cost as much as $500 each. These sensors help keep the vehicle in a very tight Air/Fuel window.
The Evaporative Monitor is also common to most vehicles. Up to 20 percent of a vehicle's of air pollution is caused by leaking raw fuel vapor from the vehicle fuel storage system—the fuel cap, filler neck, fuel tank, fuel lines, fuel vapor canister, and included valves and vacuum lines.
The EVAP monitor has some specific enabling criteria. The fuel tank must be between 15 and 85 percent full. This is because the monitoring algorithms depend on a consistent (within a range) vapor pressure. If the fuel tank level is too low or too high, the fuel tank pressure sensor data readings cannot be trusted, so the EVAP monitor will not run. The other enabling criteria are that the vehicle must be "stone-cold," meaning that the engine has been off and the key out of the ignition (many vehicles are not really "off" until the ignition key is out) for at least eight hours. This allows the engine to be within 1 to 5 degrees of the ambient air temperature, a cooling process that takes eight hours or more. Some vehicles actually run the EVAP Monitor while the engine is off. This is done by a separate, tiny computer that is dedicated to running the EVAP test. It will come on after the vehicle has been "off" for the appropriate amount of time.
The EVAP Monitor is basically a leakage test. A vacuum condition is created in the fuel tank and vapor canister—then the vacuum data is recorded. The OBD II software can detect how large a leak there is in the system. Most 2000 and newer vehicles fail the EVAP monitor if there is a .020" leak or larger. This is about half the width of a head of a pin!
Many vehicles have an EGR system monitor. The purpose of the Exhaust Gas Recirculation (EGR) system is to literally recirculate inert exhaust gas back through the combustion chambers in order to cool the peak firing temperatures to below 2500º F. This below 2500º condition prevents the formation of nitrogen oxides (NOx), which are poisonous gases. When the heat is 2500º or above, the oxygen and nitrogen molecules will actually bond and form various NOx compounds.
Most EGR systems are capable of recirculating up to 10 percent of the exhaust gas back through combustion. The EGR Monitor usually is enabled during light cruise conditions after the vehicle is fully warmed up. The EGR Monitor is known for being a two-trip monitor, meaning it must fail two times in a row on two successive startups and drives in order to set a code. The EGR Monitor also checks for maximum flow capacity during highway deceleration. This is to verify that the system is capable of delivering the 10 percent flow. Since it is done during deceleration, the driver will not be able to detect any unusual engine performance. If the "maximum flow" check was done at almost any other time, the engine would run rough and hesitate.