Emissions Fail on nct due to rev issue

[LukeLuke]

Hi, 

I failed the nct because the tester wasn't able to keep the revs at about 2800 , he said for the first 10 seconds it was perfect and I was getting the green light, then all of a sudden the revs would drop about 500 and then go back up again.

He thought it might be the lambda, then asked if the car had a maf sensor and said it could be that too 

 

I plugged in my own reader and I was getting voltage readings and showed them move as I revd 

I'm not sure but does this mean the fault is elsewhere or can they still look to work but cause issues before they actually fail?

Also when I leave the car idle it is perfect, the issue is only holding the revs steady for the test 

[LukeLuke]

Anyone got any idea? 

I also have a post cat code P0140 code , the voltage reading doesn't move so that's gone , I don't think it should affect performance

[LukeLuke]

Still no help on this?