Differences in two separate MOT Emission Tests

LukeSkywalker_2

Hi

I had an MOT about 3 weeks ago where my car failed due to corrosion on the sill so after getting it welded (which took longer for me to get arranged) the retest period was long gone, so I took it to a different place to get the MOT.

It passed but I'm surprised by the deviation between the two emission tests -

Here are the results - first set in black, second test results in red:

Fast Idle Test:

CO (% vol.): 0.01 0.138
HC (ppm vol.): 15 92

Natural Idle Test:

CO (%): 0.08 0.00

I'm not trying to complain or anything, I'm just curious as to why for example the fast idle test produced such different results

Wayne3765

Can't remember the exact readings but my car failed on emissions on it's last MOT , I booked it in to a tuning centre and when I got there it was ok the only difference was the tuning centre was a few miles further away from my home than the test centre was so the engine was hotter when tested.

Something simple like a loose / damp / oil fouled lead or a split air intake hose could cause a big difference in the tests as it will cause the engine management system to alter the ignition timing.

Crabman

As wayne says, it could depend a lot on whether the engine was hot/cold at time of testing... at least it passed - I've seen far worse results than those (I speak from experience! )