Catalytic converters filter harmful gases and pollutants from the exhaust system by chemically converting the gases into less harmful substances. The chemical reaction takes place inside the converter, and its performance remains undetected until a problem arises. A check engine light or a poor performing engine will both occur when the internal substrate of the converter breaks down and clogs or restricts the exhaust flow from exiting the system.

The Exhaust Pressure Test

A device used to test exhaust pressure employs a pressure gauge, tube and small hose. A drill is used with a high-speed drill bit to make a small hole in the exhaust system. To test the air flow passing through the converter, drill the hole before the converter to find out if pressure has built up between the manifold and the converter. Inserted the tube into the hole in the exhaust pipe, and connect the tube to the pressure gauge. If the gauge detects zero or minimal exhaust pressure, the converter is OK. However, if pressure is present, drill another hole behind the converter. Seal the first hole with screw plugs. If no pressure is detected behind the converter, it is clogged.

Other Simpler Tests

The vehicle is going to have obvious symptoms when the converter is partially clogged. It is going to perform poorly and burn more fuel. The engine will be under more duress to expel the exhaust pressure that builds up between the converter and the engine. It will rev higher with poor acceleration. A simple back-pressure test will determine if there is a blockage in the exhaust system. Placing your hand over the tail pipe (using a safety glove and large rag), plug the tail pipe and see if pressure builds. A good performing engine and exhaust flow will push your hand and plug away from the tailpipe. A partially clogged converter will limit the exhaust being expelled, and you could stall the engine by blocking the tailpipe. This test may not pinpoint the problem as the converter, if pressure is low. Other components within the internal stream of the exhaust system can also block or restrict pressure. A fully clogged converter can stop the vehicle from running at all. You may be able to start the engine, but once the pressure builds up, the restriction would act as a governor and choke the engine. Removing an oxygen sensor before or near the converter can also help you diagnose if the converter is clogged.

Considerations

Catalytic converters break down for a reason. Rich fuel mixture will purge the unburned fuel through the exhaust, which will linger inside the converter. The precious metals inside the converter will overheat and break down due to the contamination of the unburned fuel and the excessive heat. When a converter clogs and needs to be replaced, you should be aware that it is highly unlikely that the converter clogged because it failed on its own. Something caused it to fail. And by simply replacing it and not further diagnosing the problem, you're going to compromise the effectiveness of the new converter. Engine misfires and rich fuel mixture are two of the leading causes of internal catalyst failure.

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Catalytic Converters

Exhaust Pressure Tester