STI V4 Front O2 sensor replacement and wiring

Old 16 April 2019, 10:34 AM
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Join Date: Nov 2005
Location: Aberdeen
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Default STI V4 Front O2 sensor replacement and wiring

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 Iíve 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 couldnít get a stable test. The car is a Murtaya kit car fitted with a modified STI V3/V4 engine mapped with a Simtek ECU.

Iíve 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 doesnít 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. Iíve had grounding issues before on other cars when fitting lambdas and the four wire option seems to be a better option to me. any potential issues with this?

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