The C28 code is your ECM's way of saying the secondary throttle-valve actuator-an unassuming little servo-motor -isn't getting voltage or has otherwise gone amiss, so check for a bad connection there. How does the ECM control said actuator? It's pulsed until the throttle position sensor (TPS) reads right. So the first thing to do is make sure your TPS is adjusted correctly. If it doesn't check out, replace it. According to Marc Salvisberg, Internal Combustion Genius and proprietor of Factory Pro Tuning (www.factorypro.com) in San Rafael, California, removing those secondary butterflies can be a good thing if it's done right. Here's why: The secondary butterflies nose over beyond 7000 rpm, obstructing flow and choking power output in the process. But don't remove the whole shaft or you'll get a 29 error code. "If you remove the secondary plates the fuel delivery leans way out between 8000 and 9000 rpm," Salvisberg says. "That's very bad. So you need to retune the stock injection with a mappable device like the Teka SFI Module. Then, once you remove the airbox snorkel, it's a huge improvement." With careful tuning you're talking 8-9 more horsepower at 7000 rpm, with more available above that. So have at it.