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Believe swapping for a straight through downpipe will cause the CEL light to come on. If that is the second cat you mention then the CEL light fix is needed.
If i remove the second cat on an 05 sti would i need to fit a CEL fix.
Does the CEL fix only need fitting if i remove both cats
You need the CEL fix if you remove either the up-pipe or DP. IF you remove the second cat and/or the resonator (both in the centre sections) you don't need the CEL fix. I ran mine like that (before having further mods) for 12 months or so and never had a CEL problem.
No up-pipe cat on the sti,Cel fix only needed if the first(larger cat)is removed. The sensor is between the 2 cats so doesnt read anything behind the second(middle section exhaust)cat.