How to Read the HC PPM Emissions Test

by Jordan Whitehouse
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Most states in the U.S. require that vehicles undergo emissions or smog testing. These tests measure the amount of pollutants vehicles expel into the air and identify those vehicles that need to be fixed before they can be driven again. Emissions tests include the tailpipe test, which measures the amount of nitric oxide, carbon monoxide and hydrocarbons emitted. Within the tailpipe section of your test results you'll see "HC PPM," which refers to the hydrocarbon amounts.

Step 1

Locate the tailpipe section of your emissions test results.

Step 2

Find "HC PPM" within the tailpipe section. You read this as hydrocarbon parts per million, and it means the number of hydrocarbon molecules for every million molecules emitted through the exhaust.

Step 3

Determine whether you pass or fail the HC PPM part of the test. The number of hydrocarbons allowed varies from state to state, but generally for vehicles made after 1994, your vehicle will need an HC PPM reading of below 50.

Step 4

Interpret the HC PPM reading. If you fail the HC PPM part of the test, it could mean that there is a problem with the carburetor, fuel injection system or oxygen sensor, or that the engine oil is diluted or the air cleaner is dirty.

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