ASE TEST TIPS: Catalytic-Converter-Testing
BAR Specific Diagnostic Class
Here is another ASE Study guide from the Smog Repair License Class
The best Catalytic-Converter-Testing procedure in my opinion is the intrusive test, which means taking a gas reading before the catalytic converter and a reading after the cat. The only problem with that, you have to drill a hole in the customer’s exhaust pipe.
They sell kits for this, but must people, including me, probably will not drill a hole in someone exhausts pipe unless it’s their last choice and our backs are against the wall.
Catalytic-Converter-Testing is accomplished many different ways, the easiest way is to tap on the cat and listen for a rattle or broken components. The next test would be the temperature gun in which you would take a temperature reading before the cat and a reading after.
There should be at least a 5% increase in the temperature on the outlet side compared to the inlet side. This test is not very accurate, the beam of light can sometimes bounce off the metal pipe, some people recommend spraying the pipe with flat black paint for a more accurate reading.
EPA is requiring all cars built after 1996 to have a Catalytic-Converter-Testing monitor. The monitor checks the converter’s ability to store and burn oxygen content.
The monitor will test the ability of the converter by watching the upstream and downstream O2 sensors signals. In close loop the front O2 sensor should be varying between 100mv to 900mv.
The monitor will compare the front O2 sensor signal to the rear O2 sensor signal and is looking for a 70% difference from the front to the rear sensor. The rear sensor signal should be virtually flat and over 450mv. (most vehicles)
When the cat is not working as it was design to, the cat does not use the O2 to oxidize Hydrocarbons “HC” and Carbon Monoxide “CO” into water “H20” and Carbon Dioxide “C02”. Which means the O2 will leave the cat and the rear O2 sensor will be fluctuation as often has the front O2 sensor.
When the downstream Ho2S voltage signal begins to fluctuate within 70% of the upstream Ho2S signal on two consecutive trips, the ECM records a freeze frame data, sets a trouble code, and turns on the Malfunction Indicator Light, “MIL”.
Another test, which I use most often is the HC and Co2 cranking converter test, a good cat will produce more than 12% Co2, when supplied with enough raw fuel. While performing this test the HC and Co2 needs to be monitor.
Insert your analyzer probe into the tail-pipe of the car being tested, run the engine at 2,000 rpm’s or until it reaches operating temperature, turn the vehicle off and disable the ignition not fuel. Crank the engine for 10 seconds and monitor the HC and Co2.
Towards the end of the 10 seconds observe the Co2 and HC readings, a good cat should produce less than 500 ppm. If the recorded HC is higher than 500 ppm, look at the C02 readings.
If the Co2 reading is above 12%, the converter still passes,now if the Co2 reading is below 12%, the converter failed.
I usually find in my own personal experience this test is about 90% accurate.
The next Catalytic-Converter-Testing procedure is the O2 snap test which I personally don’t use often.
Get the engine to operating temperature, wait for the gasses to stabilize, the O2 should be at or near 0%.
If it is higher, check the CO%. If there is CO, the converter is not operating. In other words, the CO is 1.0% and the O2 is 4.56% for example, this is a sign of a bad cat.
When the CO is at least .5% and the O2 is higher than the CO, this could be a sign of a bad cat, but remember check the basics and confirm the vehicle is in fuel control before testing the Cat.
If the CO is 0% the mixture is too lean to test the converter; add propane into the intake until the CO is about .5%. If the O2 drops to 0%, the converter is not operating properly.
Once the emissions stabilize, snap the throttle wide open and let the vehicle’s rpm return to idle, now this part is a little hard if your analyzer can not record the emission readings.
After you snap the throttle, the readings will increase momentarily after the snap. When the emissions peak look at the O2 and CO. The O2 should not rise more than 1.2% when the CO peaks.
If the O2 remains below 1.2%, the converter passes.
If the O2 rises to about 1.2%, the converter is weak
If the O2 rises pass 1.2%, the converter is bad
Remember with this test the exhaust has to be leak free, and the air injection system disable.