The tenth mode of diagnostic scan tools is a rather recent invention. This mode gives emissions-related trouble codes, and essentially serves at a shield against tinkerers trying to circumvent emission laws.
The tenth mode of OBD II scan tools is not officially called Mode 10. It’s formal name is actually Mode $0a, but since it’s the tenth mode, and many people in the industry call it “Mode 10”, we’re going continue calling it that too.
Mode 10: Permanent Emission Codes
Mode 10 (or Mode $0a if you prefer) is essentially another listing of stored codes that have caused the “check engine” light to come on. Unlike codes found with Mode 3, these codes cannot be erased. Neither Mode 4, which is used to erase error codes, nor the basic technique of unhooking and reattaching the battery, will erase the codes.
The mode was put into place in 2010, with full usage by 2012. The goal was to counter individuals who shut off the check engine light before an emissions check. Prior to 2010, disabling the light would result in no error codes, even if the vehicle had an issue. This essentially gave the vehicle a free pass during an emissions inspection.
With Mode 10, however, codes that fall under the mode’s jurisdiction will only clear when the monitor a full cycle, which it can’t do if the light is disabled. Codes that fall under Mode 10 are often called “permanent codes”, because they are stored in the computer for a long timeframe, making them, (as far as emissions tests are concerned) permanent.
Strengthening Emissions Tests with Mode 10
Emissions tests, while not used in every state across the country, are an important practice for reducing smog and air pollution. Especially in heavily populated areas like Los Angeles, Dallas, or Boston, where tests are mandated, Mode 10 is an important function for maintaining the integrity of these inspections.
Mode 10 is a recent function, but it will likely be seen more and more, especially if new states decide to mandate emissions inspections.