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The engine warning light is constantly on. The local agents workshop say they scanned vehicle and found the converter needs replacing, costing $2000+. A local mechanic tells me it could be other things such as an oxygen sensor causing the P0420 code, and the only way to tell the converter needs replacing (is damaged or clogged up) is to take it off and physically inspect it.
Who should I believe here? The car runs very well, got the usual power, done 135 000 km

The P0420 code in a Suzuki Swift will often be thrown one of your oxygen sensors are not functioning properly. Typically, the code will be triggered by the upstream oxygen sensor (before the converter).

It may be worth a trip to Suzuki for a diagnosis before spending too much Money, if you do end up replacing any sensors go genuine the aftermarket ones rarely work property especially upstream sensors known as (AFR) or air fuel ratio sensors.

The converters seldom fail, and if you remove the upstream sensor you should able abele to give the catalytic converter a quick check.

Many thanks Ian. So if I get a Suzuki diagnosis they will check if the oxygen sensors are working properly, and that the converter itself is the problem? I don't want to pay $$$$ for a new or replacement converter if it's a much cheaper problem. The Suzuki service people really don't like giving the full story, which is why I posted this question).
Thanks