Repair Info - Emissions Repair Explained
Catalytic Converter Testing
Catalytic converters are also referred to as cat converters or cats. There is only one way to conclusively check the performance of a cat: before-and-after testing. Other methods are not precise enough to measure the difference between a catalytic converter that is operating at 75% efficiency (bad cat) and one that is operating at 95% efficiency (good cat). Unfortunately, on certain vehicles, this test is not possible due to the location of the catalyst and alternate test methods are required.
A qualified technician can quickly determine cat efficiency by measuring the exhaust emissions ahead of the cat, then immediately at the tailpipe, and comparing the two sets of readings. To be a valid test, the engine should be warmed-up and run at 2500 rpm for two minutes to ensure the cat is at normal operating temperature before making the measurements.
Guidelines For Acceptable Catalyst Efficiency
For vehicles 1991 model year and older, the catalyst should be at least 80% efficient for HC and CO. For vehicles 1992 model year and newer, catalyst efficiency will need to be 90% efficient or better for HC and CO. This means that the HC and CO that comes out of the catalytic converter should be 90% less than what goes in.
The arithmetic is pretty basic. The difference between engine-out emissions (gases going into the cat) and tailpipe emissions (gases coming out of the cat) will give the technician the amount of reduction. To get efficiency in percent, divide the amount of reduction by the amount of the gas going in, and then multiply by 100 to arrive at the percent reduction. The formula and an example using HC emissions is shown below:
Catalytic Converter Efficiency Formula
(engine out level - tailpipe level) ÷ engine out level × 100 = efficiency in percent
Catalytic Converter Efficiency Calculation Example
Measured HC upstream of catalytic converter = 80
Measured HC at tailpipe = 4
(80 - 4) ÷ 80 × 100 = 95% efficiency
