Tech Tip: What are the symptoms of a clogged catalytic converter?

by Ben Pila, Jr.
Member at Large

The most noticeable symptom will be a lack of power. You step on the gas and your Z car doesn’t get up and go. A badly plugged converter may not even let the vehicle go over 30 mph. The converter will be hotter on its intake side if it is plugged and may even have a dull red glow if the engine has been running a while.

Here’s the simple procedure for testing for a clogged catalytic converter:

1. Connect a vacuum gauge to a vacuum line fitting on the intake manifold. The easiest access points are either off the vacuum fitting going to the brake booster check valve or the vacuum hose fitting for the charcoal canister purge valve.
2. Record vacuum at idle (usually over 17 inches of Hg).
3. Record vacuum at 3000 RPM with engine at no load.
4. The vacuum reading taken in step 3 should be at least as great as that taken in step 2. If the vacuum reading in step 3 starts dropping (even going positive) from it original reading, then you could have a clogged converter.

This is an indirect test - a better one is to use a backpressure tester. This is a vacuum/pressure gauge with an adapter, which allows the tester to be screwed into the oxygen sensor’s socket.

At 3000 RPM, the backpressure should be below 3 PSI. You can buy these at your local automotive parts place for $25-30.