2002 Volkswagen Golf Catalytic converter or sparks?

Engine Performance problem
2002 Volkswagen Golf 4 cyl Manual

My car's "check engine" light came on flashing the other day. My VW manual lists a flashing engine light as being possible catalytic converter damage. The car still starts, but with less power, and does not accelerate well. It is a little noisier than usual, and a lot more sluggish. If I drive up a slight hill, with my foot pressing the gas pedal all the way down, I can barely get past 15 mph; I can't drive it as normal. Only drove it once since the engine light came on. The mechanic I took it to tested the car & said it's only working on two cylinders (it's a 4-cyl. Car). He did not mention catalytic converter; just said it's possible spark plugs and/or coil damage.
At what point do I get my catalytic converter looked at/tested?