Verifying an Excessive Grams Per Mile (GPM) Indication Using a Parts Per Million (PPM) Reading

For vehicle gas reading(s) that are excessive, compare the actual gpm reading to the gas cutpoint level needed to pass testing. This gives an indication of how much the ppm reading has to be reduced. If the actual reading is twice the cutpoint, the baseline reading has to be cut in half or more.

Example:

• The actual hydrocarbon (HC) produced by a vehicle was 1.6 gpm. The cutpoint for HC in this example is 0.8 gpm. The actual reading is twice the cutpoint.
• The HC reading obtained for the same vehicle during the baseline drive averages 440 ppm. In order for this vehicle to pass the I/M test, the HC reading from the verification trip must be at least half of the baseline reading, or an average of 220 ppm or less.
• This method only gives a general idea of how much the ppm reading needs to be reduced in order for the vehicle to pass an I/M test that calculates gpm. This test is not exact. Experience still has to be used to determine if the emission readings are reduced enough for the vehicle to pass the I/M test.