have noticed that lean ups the ratio of another gas .. cant remember which one..
this will sound bad, but to pass a CA18 in a sprinter we had a copper pipe attached from the turbo outlet pipe, onto the exhaust pipe..
so it basically pumped fresh air into the exhaust ready for them to read
i gues that would suffice if your desperate... just know how much Moanie on teh forums had to go through with Hitman to retune the car over and over to get it to pass and that was on a 240rwkw RB25