Cats started to trickle on to the UK market in 1988. They arrived en masse in 1993, when converters became essential accoutrements for petrol- powered cars. Nothing else would get them through tougher emission regulations.
Next January, when the first of these three-year-old cat cars face their first annual test of road worthiness, the MOT inspectorate will be waiting with a rewritten rule book.
Like people, cats deteriorate with age. Physical damage and contamination, caused by persistent back-fires, air leaks, oil smoke or the use of leaded petrol, can wreck them.
Not that the driver would know. Neglected high-mileage cats will pass the current MOT emissions test "without breaking sweat", according to the AA's Rayner Peett.
Designed as a crude pollution monitor for non-cat cars, the present exhaust-probe test is a token green gesture. "It is incapable of telling anything about a catalytic converter's performance," says Rob Searles of Johnson Matthey, the world's biggest supplier of exhaust gas converters.
The DoT is pragmatic. "Older catalysed cars are not regulated," says David Stewart, a DoT spokesman. "They are not required to meet special standards." But what of the new test? Is mass failure a likely outcome? Could it put older cat cars off the road? According to the AA's Mr Peett, the cost could be greater than the car's net value. "There's no definitive evidence about how long catalysts last," he said. "For many people, the need for a new one could be a nasty shock."
The impecunious should not expect a cheap fix at their local Kwik-Fit, which does not stock off-the-shelf cat replacements. The franchised dealers seem to have a monopoly.
The message from the Consumers' Association is encouraging. "Our annual car reliability surveys have not shown any unexpected expenditure on exhaust catalysts," says Liam McCormack, for the association.
Mr Searles dismisses alarmist talk. He asserts that the new MOT test will detect gross malfunction but not specific emission levels. "It might indicate a major systems failure, but that's all," he says. "Catalysers should not be treated in isolation. They are part of a system that also involves the car's injection and ignition."
The news gets better. Tests conducted in Germany indicate that failure of a car's emission control is more likely to be caused by problems under the bonnet, by fuelling or control faults. "The cat is at the bottom of the list. Engine tuning will normally rectify problems," says Mr Searles.
Johnson Matthey is confident that motorists have little to fear. If you have an old, high-mileage cat-car that has been well maintained, it should pass.
The bad news, especially for the environmentally concerned, is that the new test will not show whether your catalyst is meeting its design objectives and reducing carbon monoxide, hydrocarbons and nitrogen oxides by up to 90 per cent.
"The US is only just getting to grips with this problem after two decades," says Mr Searles. He refers to what is known in the States as IM240 - an inspection and maintenance test lasting 240 seconds (four minutes) that is compulsory in some designated high-pollution urban areas. Computer analysis of the exhaust gases shows whether the control system, of which the cat is a part, is working effectively.
No such test exists here. "We're moving very slowly," says Mr Searles, who would like to see something like IM240 introduced here.