Hi my engine management light isnt illuminated at engine start up, it's failed it's MOT  due to high co2 emissions and inop mil light, dad's run an obd reader and it says p0420, can anyone help as I need get this light working, we've took instrument panel out and there's no signs of tape on led or any tampering, we are at a loss

Hello Samantha,

what engine have you got ?

does the airbag warning light come on at start-up and then go out ?

P0420 is a very basic generic code and the cause could be many things. You need to use the software FORScan to see if there are any Ford specific codes.

In the time you have owned the car, have you ever seen the EML on at start-up ?

Hi, you'll also need to fix the high CO² emissions too! 

Start by taking out the air filter and looking at the underneath of it -(that's the side that gets dirty) you'll be surprised how many ppl neglect to check/change the air filter. 

The code may be an old one caused by a condition that is no longer upsetting the ECU.  For instance, it will alert you to misfires, but extinguish the light if you complete two trips without further misfires.  If readings tell it that a certain condition no longer exists, it will extinguish the light, but the code will still be stored.

If the light illuminates when your ignition is on, but the engine has not yet been started, then it will be working.  I think I read that it goes off once it gets an RPM reading from the crankshaft position sensor; so if it comes on when you switch on, then goes off, then the ECU is at least talking to it properly.

Do you mean CO (carbon monoxide), rather than CO2?