Tech Article: Catalytic Converters
by Larry Carley
Tire Review, December 1996

It's been awhile since we've covered an exhaust topic in this column, so let's look at the one item that's the most profitable to replace: the catalytic converter.

Every vehicle built for the past 20 years has one. Original equipment catalytic converters are designed to last the "life" of the vehicle. But many converters never go the distance 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 ignition or mechanical problems that cause the converter to overheat.

Converters are essentially afterburners that re-burn 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 extra 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), or silicone deposits from internal coolant leaks (cracked head or block, or a leaky head gasket). As the contaminants build up, HC, CO and NOX emissions begin to rise. On a late model, well-tuned engine with a properly functioning converter, HC and CO emissions should be nearly zero. So if there's a jump in emissions, it usually means its time to replace the converter.

There's no way to rejuvenate a dead converter, so replacement is the only repair option. Up to model year 1995, converters were covered by a 5 year/50,000 mile federal emissions warranty (7 years or 70,000 miles in California). In 1995, the warranty jumped to 8 years and 80,000 miles.

The EPA's rules for replacement are quite strict: you can't replace a converter until it is out of warranty and a legitimate need for replacement has been established and documented (such as a blockage, failure of an emissions test, or to replace a converter that someone removed). You must also obtain the customer's authorization for repairs in writing, keep the paperwork for six months and the old converter for 15 days. The replacement converter must be the same type as the original and installed in the same location.

How do you know if a converter needs to be replaced? An emissions test will tell you if the converter is doing anything or not. Low intake vacuum readings are a symptom of excessive backpressure which may be due to a plugged converter. A complete blockage will prevent the engine from running after it starts.

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.

If you're replacing a plugged converter, it's important to remember it failed because of overheating. Replacing the converter will eliminate the restriction, but the new converter will likely suffer the same fate unless you identify and repair the problem that caused the converter to overheat in the first place. Look for things like fouled spark plugs, bad plug wires, a cylinder with low or no compression, or a computerized feedback system that stays in open loop all the time.

The oxygen sensor may need also need to be replaced. To do its job efficiently, the converter needs an air/fuel mixture that is constantly flip-flopping back and forth from rich to lean. If the O2 sensor is sluggish or dead, the fuel feedback loop will either flip-flop too slowly or remain rich all the time. Either way it will increase CO emissions and converter temperature.

A bad coolant sensor that always indicates a cold engine (or an open coolant sensor circuit) will also keep the fuel system in open loop, which means a steady diet of excess fuel and poor converter performance. A thermostat that's stuck open or is too cold for the application can also cause the same sort of problem.

It's also important to check out the air pump (or aspirator valve) and related plumbing since these components provide fresh air for the converter to re-burn the pollutants in the exhaust. If the air pump isn't working, or the air isn't getting to the converter in the right amounts at the right time due to a bad diverter valve or damaged or leaky plumbing, it can reduce the converter's operating efficiency significantly.

Background Information

What is a Catalytic Converter?

A catalytic converter is a device which cleans up the exhaust gases of a vehicle. The catalyst does this by enabling certain chemical reactions to occur under the conditions within the vehicle's exhaust system. Vehicle catalysts convert poisonous gases into harmless ones by the reactions below:

CO (Carbon Monoxide) --> CO2 (Carbon Dioxide)
HC (Hydrocarbons) --> H2O (Water) + CO2
NOX (Nitrogen Oxides) --> N2 (Nitrogen) + O2 (Oxygen)

The catalyst promotes the above reactions, which in the case of the first two involve the addition of oxygen (Oxidation), and in the case of the third reaction involves the removal/release of oxygen (Reduction). The active components of exhaust gas cleansing catalysts are usually Platinum and Rhodium, although sometimes Palladium is used instead of, or in conjunction with, Platinum.

How Long Have Catalytic Converters Been Used In The UK?

Some manufacturers began fitting 'cats' as early as 1989, although they were not compulsory on new cars until 1st January 1993. This compulsory fitment was backed up by a new, more stringent, MOT emissions test applicable to all cars registered on or after 1st August 1992, and fitted with a cat

In theory a catalytic converter will last indefinitely - however....In practice catalysts have to be replaced as a result of physical damage. At the center of most catalytic converters is a ceramic core, which can become very brittle. Any sudden mechanical shock may crack the converter core - after which it will rapidly disintegrate. Incorrect functioning of other engine management systems: A faulty fuel injection or ignition system may give rise to excess unburned fuel in the exhaust system. This will ignite on contact with the hot catalyst and destroy the reactive surface. In extreme cases, the ceramic core may even melt thus restricting gas flow. This can also happen if the car is ever 'bump-started'. Often failures are only detected during routine servicing or when the vehicle is MOT tested. However a catalyst which has become blocked, due to cracking, meltdown, or obstruction with engine debris, may be hard to detect. Warning signs include poor performance and increased fuel consumption - which may justify replacement of the cat even if the vehicle is still meeting the necessary emission standards.

Looking After Your Cat
To maximize the lifespan of your catalytic converter you should:
Never use leaded petrol or fuel additives containing lead.
Have your car properly serviced in accordance with the manufacturer's instructions.
Never bump-start your car (use of jump-leads is acceptable).
Avoid bumping the exhaust on rough ground, curbs etc. The cat may even be damaged if you reverse into a wall and the shock is transmitted through the exhaust system.
Investigate and correct any rough running of your car without delay.

It is however inevitable that failures will occur over time.