You need to check for two possible conditions: contamination and restriction.

The catalytic converter is one of the most important emission control devices on a vehicle today. Every car and almost every truck that's been built since 1975 has at least one converter in its exhaust system. So knowing how to inspect and diagnose this important device is an important aspect of exhaust system service.

Original equipment converters are designed to last the life of the vehicle, which in most cases means 100,000 miles-plus. Federal emission warranty requirements are now eight years or 80,000 miles on the OEM converter. This changed from five years or 50,000 miles back in 1995.

Many original equipment converters do last the life of the vehicle, but many also fail far short of the 100K mark for a variety of reasons. Some succumb to rust and corrosion. Others are "road kill" victims, damaged by pot holes or other objects. The most common causes of failure, though, are engine problems that cause the converter to overheat, and contamination.

Converters are essentially afterburners that reburn carbon monoxide and hydrocarbons with additional oxygen from an air pump or aspirator valve. Three-way converters also break down oxides of nitrogen (NOX) into nitrogen and oxygen. All this produces a lot of heat in the exhaust, which the converter can handle as long as there are "normal" amounts of these pollutants in the exhaust. But when too much unburned fuel enters the exhaust because of a misfiring spark plug, overly rich fuel mixture, a leaky exhaust valve or head gasket, the converter's operating temperature can soar causing the converter's innards to melt resulting in a 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 the accumulation of phosphorus deposits from oil burning (worn valve seals, guides and/or rings), silicone deposits from internal coolant leaks (cracked head or block, or a leaky head gasket), and/or sulfur deposits from trace amounts of sulfur 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. So if an emissions test reveals higher than normal emissions, the converter may need attention.

One way converter life is being extended is with the new "SJ" rated motor oils. SJ rated motor oil is a "low polluting" motor oil that contains less phosphorus than earlier SH rated oils. The difference isn't much, a maximum of 0.10 percent phosphorus in SJ rated oil versus 0.12 percent in SH rated oil, but the American Society of Testing and Materials (ASTM) says that lowering the phosphorus content of the oil lessens the risk of phosphorus contaminating the catalytic converter.

There's no way to rejuvenate a dead or damaged converter, so replacement is your only repair option. But if the converter is only mildly contaminated there are aftermarket products that may help clean the converter and extend its life. One such product is a three-part cleaning treatment that consists of one can of cleaner for the fuel tank, one can of cleaner for the crankcase, and a can of solvent which is fed into the engine through a vacuum hose while the engine is running. The product's supplier says the combustion byproducts from the three cleaning agents combine with the unburned solvent to oxidize and loosen hydrogen sulfide and other deposits in the converter. The result, they say, is a dramatic improvement in engine performance and lower emissions.

DIAGNOSIS

You need to check for two possible conditions: contamination and restriction. 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. But before you condemn the converter, be sure to check the air pump, diverter valve, aspirator valve and all related plumbing for leaks, blockages or other problems.

On 1996 and newer vehicles 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.

Checking for restrictions is easier and doesn't require any special equipment. A restriction problem might be suspected if your customer says his engine lacks power, has been using a lot of gas lately, or stalls after it starts and won't restart.

A low intake vacuum reading is a classic symptom of excessive backpressure which may be due to a plugged converter. If the vacuum reading drops and the engine stalls, the converter may be plugged.

Note the reading at idle, then hold rpm at 2,500. The needle will drop when you first open the throttle, then stabilize. If the reading then starts to drop, backpressure is building up in the exhaust system.

You can also attempt to measure exhaust backpressure directly. If the car has air injection, disconnect the check valve from the distribution manifold, and insert a pressure gauge. Or, remove the oxygen sensor and take your reading at its hole in the manifold or headpipe. Refer to the backpressure specs for the application. Generally speaking, more than 1.25 psi of backpressure at idle, or more than 3 psi at 2,000 rpm tells you there's a blockage.

A "thunk" test on the outside of the converter with a soft rubber mallet will tell you if the catalyst inside is loose. Older GM pellet-style "bedpan" converters should rattle, but monolithic converters should not.

If you suspect a blockage, disconnect or remove the converter and look inside with a trouble light. If you can't see through the honeycomb, the converter is obstructed and needs to be replaced.