Engine light on - help?

Hi guys new to the forum and cars so looking for some help.
I got a k11 Micra (2001) in September off a family friend mechanic. I needed it urgently and after failing the NCT (Irish version of an MOT) on emissions the first time it passed the next day, I think he did a quick job in order to pass it.

Engine light has been on for about the past month, I checked it with a scanner and it gave the codes:
PO137 , PO171 , PO420 . Performance hasn't dropped since being on and I was told that these were just codes for emissions, presumably as the mechanic just did something to scrape it through the test.
I had the battery out recently while doing some work and the light stayed off for 250 miles before returning.

Can anyone confirm what the issue could be? As said I have very little knowledge.
Many thanks for any help.
 
po137 bank 1 low voltage exhaust leak or wiring short to earth

po171 system too lean bank 1 intake exhaust leak air system maf sensor fuel pressure punp injectors ho2s

po420 cat wiring h2os2

check the landa sensor wiring or change it for another supermarket fuel kills thease !!!!
 
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