It's my understanding that failed emissions tests (CT's to be specific) usually come from sensors showing error/not ready codes and that a mild tune such as to adjust for an exhaust or intake change that doesn't mess with those sensors should be fine (cost/benefit arguments of those kinds of mods aside). But there're plenty of people saying ANY ECU tune will cause a failure. I suspect some of these posts are from cat deletes and ECU's tuned crazy/poorly to override/ignore CEL triggering conditions. Or am I wrong, and would a quality "reasonable" re-map really auto-fail?
If you have a tuner that knows what they are doing, you can pass emissions. If you use someone like Bama Tuning who just does email tunes, you're going to automatically fail. And here is why:
Tuners who tune your car via email have a standard template they follow for tuning. If you tell them you have long tube headers, for example, then they will simply turn off your rear 02 sensors. This means you will automatically fail inspection because you will have two cycles showing "Not ready": Your 02 sensors and your 02 sensor heaters.
If, on the other hand, you have your car tuned in person at a shop, we can dial in your tune to where it will pass inspection. Typically if you live in an area that does inspection, we will tune the car specifically for passing inspection. You load that tune two days or so before you plan to have it inspected and drive the car through its drive cycle. The inspection tune will turn on the rear 02's, etc. but dial back horsepower and run a bit rich so that you won't trip any emissions lights. After you pass inspection, load up the performance tune and boom....full power returns, 02 sensors off, no CEL.
So, in short, if you have a tuner who knows what he is doing, he can get you through inspection. Just avoid the canned email tunes.
*NOTE: Any change WILL void your vehicle warranty automatically though as any changes can be detected. If you don't have a warranty or don't care, then tune away.
Thanks @mod_man !
I guess what I was trying to confirm is if there is a point to which you can adjust the ECU from stock that won't fail emissions without having to overhaul for inspection. Or (performance gains aside) just changes to make sure the car runs at level (example: making an adjustment for replacing cracked plastic OEM intake tubing with a metal non-OEM pipe). Based on your response it seems like it is not impossible, albeit that tune would be at the very low end of performance gains.
You are correct. For repairs like that, it would be a very simple tune and just about any shop should be able to do it for you at a very reasonable cost.