Re: 6 ohms and 0.45 volts?
cats usually rattle as they get older as the porcelain breaks up. if you get the cat off the car hold it up to the light you should ONLY jsut be able to see there is infact light at the end of the tunnel. That was on a 20k mile cat though so i guess as it breaks up it will become less dense.
Wouldnt a good way of testing the cat be to get a cheap emissions tester measure it at the exhaust tip then whip the cat off and measure from the downpipe.
Regarding the revs dropping when holding high mine has done that occiasionally ive never figured out what did it and of course its never done it when ive then got home to look at it. Have a look at the throttle body - whip the inlet pipe off and use some petrol and clean all the unburnt deposits from around the throttle flap make sure it can move and freely and does not catch.
if you hold the throttle valve open a bit from under the bonnet can you see iff the stepper motor is intervening when it's fluctuating rpm?
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