A catalytic converter decreases the toxicity of released emissions from an engine. A GMC Sierra catalytic converter can develop problems that can increase emissions, but these can often be resolved via some basic troubleshooting techniques.
A rattling noise emanating from your GMC Sierra catalytic converter can result from loose heat shields, caused by rust buildup around the welding. Check the heat shields for looseness, and re-weld them or install a clamp to secure to the catalytic converter.
If your GMC Sierra suffers from loss of engine power, the catalytic converter may be restricted. Remove the bolts on the catalytic converter flange and detach the converter. Position a light at one end of the catalytic converter and look through the other end. If you see partially obstructed or no light, the catalytic converter is clogged and requires replacement.
Exhaust back pressure, or emissions from the exhaust backing up into the engine, results in poor fuel economy or engine stalling. Check the intake manifold vacuum with a vacuum gauge while the engine idles; if you read less than 18 inches of vacuum, back pressure is building in the exhaust system. Inspect the converter for a crushed or otherwise damaged exhaust pipe, and replace it as needed, or replace the catalytic converter if it is restricted.
