How to Understand Trouble Codes
Many vehicles have a Secondary Air System Monitor. The Secondary Air System is a form of air injection into the exhaust system in order to "after burn" CO and HCs. Most of the modern air injection systems run only during cold or cool start up and most vehicles have some sort of air pumping system to push fresh air (oxygen) to help burn the dirtier exhaust during cold/cool engine operation. When the vehicle is warmed up, the exhaust system and CAT are much more effective and the additional, or secondary, air is no longer needed to bring the tail pipe emissions into compliance.
Some vehicles have a Catalytic Heater Monitor, which is similar to the Oxygen Sensor Heater Monitor, only on a much larger scale. These vehicles have a large heater element inside the Catalytic Converter(s) to heat the CAT on cold/cool start up and, in some cases, keep the CAT hot during long idle durations. Usually there are up to two dedicated batteries to heat the CAT(s) because the current is in the hundreds of amps. The normal vehicle charging system and battery could not handle this enormous electrical load. Almost all of the vehicles that have these types of electrically heated converters are powerful, exotic 10 to 12 cylinder vehicles with horsepower ratings of 350 and more. There is so much fuel being burned that the only way to keep them in compliance is to have extreme emissions equipment.
A few vehicles have an Air Conditioning Monitor, though it's not very common. There are few reasons why this monitor exists. It was discovered that some vehicles were polluting when the air conditioning system was turned on due to the 20+ horsepower load put on the engine from the air conditioning compressor. Vehicle manufacturers actually had to change the engine management to keep the engine idle smooth. They did this by making the engine run richer to the point where it was polluting when the A/C was engaged. Another reason was to check for CFC pollution. If the A/C system did not hold a certain level of pressure, then it was known that the refrigerant had leaked out into the environment, which hurts the ozone layer.
Most of the vehicles with an Air Conditioning Monitor have the A/C system on all the time. The vehicle operator has to manually turn off the A/C system to allow the A/C Monitor to run.
Mode 6 OBD II Data Stream for the Non-Continuous Monitors
Briefly, this is a way for technicians to "witness" the actual scientific test output data of the non-continuous monitor group. Each monitor has many tests, which are run in several sequences. Their output is usually in a hexadecimal numeric data stream. If a technician wants to drill down to the finer points of the cause of a monitor failure, there are ways to access the data conversions from the manufacturer to see what part of the system or component is failing and in what stage.
For example, the oxygen sensor may be voltage-high and low-range responsive, but it may be a few milliseconds slow for a particular OBD II system. The Mode 6 data stream would show this. After many, many miles of driving and cold starts, a monitor may not set a trouble code—the monitor is considered "hung." The Mode 6 data can bring to light what part of the monitor testing is close to, but not quite, failing. The monitor will run almost endlessly without passing or failing, sometimes for months and hundreds, even thousands of miles. This can be a huge problem for someone trying who needs his vehicle to pass an emissions test and the vehicle keeps failing the OBD II functional portion due to a "not ready" monitor(s) status.