STI V4 Front O2 sensor replacement and wiring

[NickMurt]

So my car failed the MOT last week on emissions. The car has been off the road for a couple of years and this was its first MOT since I’ve owned it. The MOT tester did try a few times, but the Lambda, HC and CO levels were apparently jumping about too much and he couldn’t get a stable test. The car is a Murtaya kit car fitted with a modified STI V3/V4 engine mapped with a Simtek ECU.

I’ve since fitted a wideband gauge to check the AFRs. On initial start-up the AFR seems OK (between 14 and 14.8) but very quickly goes rich (circa 11.5 at idle). The same happens when I increase the revs to mimic the fast idle emissions  test (AFR goes to ~14.5 then drops to ~12)

So looks like the front Lambda needs to be replaced. The installed sensor is a 3 pin Bosch unit, but appears to have a non-standard triangular shaped plug.

Since there doesn’t seem to be a OEM part with the correct plug available what would be the best replacement?

Based on google research it appears that the Bosch units are not recommended, for these cars,  so was planning to go for a universal Denso sensor and change the connector. Would this work or do I need to get the OEM part.

Was also considering switching to a 4 wire sensor and running a new sensor ground. I’ve had grounding issues before on other cars when fitting lambdas and the four wire option seems to be a better option to me.

I have also tested the coolant temp sensor, which seemed ok, but I will replace it anyway.