My 1998 Fiesta (1.3 Endura-E) has just failed its MOT on emissions:

Fast idle test: CO 0.65% (too high, fail), Lamdba 1.038 (too high, fail), HC 105ppm (pass)

Natural idle test: CO: 0.5% (pass)

The engine was nice and warm during the test, and I believe the tester wanted it to pass. Things I did before the test:

New air filter, oil change and new oil filter, checked valve clearances, cleaned muck from inside rocker cover, checked exhaust system for leaks, ran car at high revs for a couple of miles. The cat (around £250 on this model) was replaced 11 months ago.

My next step will be to check the output of the oxygen sensor with a voltmeter, and peer at the internals of the cat though the hole.

Changing the spark plugs is a last resort, because they will probably snap when I try. It runs fairly smoothly, with no obvious misfires. This is a rusty 13 -year-old car, so if I can't fix it for under £500, I will probably replace it.

Any suggestions on what else I should do?