Ive now taken the decat pipe to the next level on my 172 - fooling the ECU into thinking that the rear lambda sensor (downstream) is working and I have a cat fitted to the car !
The rear lambda sensor has been removed and the 2nd hole in the decat pipe plugged. A resistor has been placed inline between the ECU and wiring loom to the lambda sensors.
Initially the resistor was going to be placed between rear sensor and wiring loom but this wasnt possible (earth problem or something) - thus sensor removed, etc.
First 24 hours after mod. I had two occasions where I was starting the car from cold and the engine turned over several times before firing - which was rather concerning as it has always done it on first fire.
Since then the engine has fired up as normal on cold. It seems a little lumpy on idle to start with.
Im still in two minds whether to bin the decat pipe off and go back to the cats. Decat gives better mid range pull and car doesnt feel strangled top end.