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Diagnosing a Faulty Catalyst (Catalytic Converter)

Created on 2011-03-04 by IPD Staff

Diagnosing Faulty Catalytic Converters

Over the years we've had several older Volvos (1981-1990) require new catalytic converters to pass local emissions testing. We’ve found that the enhanced emission testing performed on a dyno in most states for cars 1981 and newer, is much tougher to pass. Often a new catalytic converter is the only way to get the car to pass. In the past it was possible to pass emissions test with a properly tuned car, however the newer procedure tests under load and the catalytic converter must be working properly to reduce emissions to acceptable levels.

Converters function like an incinerator, burning excess emissions, which produces a lot of heat in the exhaust. The converter can handle this as long as there are “normal” amounts of pollutants in the exhaust. When too much unburned fuel or other contaminants enter the exhaust because of a misfiring spark plug, overly rich fuel mixture, or a leaky exhaust valve or head gasket, the converter's operating temperature can soar, causing internal meltdown. This results in partial or complete blockage.

Converters can also fail because of contamination. As a converter ages, the catalyst gets “tired” because of a gradual accumulation of contaminants on its surface. The process can be accelerated by oil burning (worn valve seals, guides and/or rings), internal coolant leaks (cracked head or block, or leaky head gasket), and sulfur deposits normally found in gasoline.

As contaminants build up inside the converter, HC, CO and NOX emissions rise. On a late-model, well-tuned engine with a properly functioning converter, HC and CO emissions should be nearly zero. If an emissions test reveals higher than normal emissions, the converter may need attention. There's no way to rejuvenate a dead or damaged converter, so replacement is your only option.


An emissions test is the best way to check for contamination by measuring the levels of HC and CO in the exhaust (and NOX in areas that test for this gas, too). An outright emissions failure and/or higher than normal HC, CO and NOX readings on a properly tuned engine in good condition would tell you the converter isn't doing much to reduce emissions. On 1996 and newer models with OBD II diagnostics, a contaminated converter will cause the Check Engine light to come on if the readings of the upstream and downstream O2 sensors reveal little or no change in oxygen levels as the exhaust passes through the converter. A P0402 trouble code indicates low converter efficiency.

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