Non-Methane Hydrocarbon Catalyst (NMHC) Monitor
Some diesel vehicles include an extra catalyst called a Non-methane Hydrocarbon Catalyst. It’s designed to remove non-methane hydrocarbons (NMH) left over from the combustion process from the exhaust stream.
The NMH catalyst materials react with the exhaust gases to convert CO, HC, Particulate Matter and Soluble Organic Fraction into CO2 and H2O.
The computer checks the efficiency of the catalyst by monitoring the quantity of NMH in the
exhaust stream. The monitor also verifies that sufficient temperature is present to aid in particulate matter (PM) filter regeneration.
The NMHC Monitor is only used on diesel vehciles.
The NMHC Monitor is a “Two-Trip” Monitor. If a fault is found on the first trip, the computer temporarily saves the fault in its memory as a Pending Code. The computer does not command the MIL on at this time. If the fault is sensed again on the second trip, the computer commands
the MIL “On,” and saves the code in its long-term memory.