Right I took my car for it's mot and it came back as CO and HC emissions too high. So I took an educated guess that something is causing it to run rich. Plugged in my obd and I got fault P0036. It also shows sensor 1 reading a voltage of between 0.1 and 0.8 volts, whereas the sensor 2 voltage is take consistently at 0.405 volts. Am I right in saying the problem is my sensor two which is located under the car? Would a normal lambda sensor be OK to swap with it as I know the Haynes manual refers to it as a different name? Thank you

whats the lambda reading on the mot emmisions sheet?

What was the Lambda reading on the emissions test?

Edit - Lol...  Echo in here.

great minds think alike. lol

Lambda reading was 0.999 and 1.014... I replaced lambda sensor 2 and the fault code disappeared, a load of white smoke came out for ten minutes and then cleared. I am booking it in for a re test but I have been told to prepare to be disappointed as it sounds like the catalyst isn't performing correctly. I read somewhere sensor 2 may fine tune the fuel map a little bit and my initial thoughts it's causing it to run rich. Which kind of threw me because the lambda reading from the mot centre was bang on 1. I absolutely love fixing cars but my bank account doesn't haha

Yeah it does sound like the cat unfortunately, get it really hot before the test if you can.  

the downstream sensor you have changed has no effect on fueling, the upstream sensor does the fueling correction. so you have fixed one fault but from the readings you have posted, it does look like cat is not efficient anymore.

when ide got my Scooby with nocat  on it I used to go for a good burn out  and a bottle of emissions  reducer you can get it from any good garage and allways flew though