Our 2004 Hyundai Santa Fe with the 2.7L V6 and auto transmission has been a great car for us, with 230,000 miles of almost exclusively high-speed driving so far and no major problems. But now it won't shift out of 2nd gear. One mechanic we know says it might be a sensor problem, another says it is because the catalytic converter is shot (it is original equipment and never been replaced). Neither shop had the equipment to check the error codes, and the AutoZone people said they couldn't read any codes. We've checked the trans fluid; the levels are good, there are no leaks, the fluid is clean and we have never had trans problems. We also have never replaced the fluid because we've been told it could do more harm than good replacing the fluid after 100K. The "check engine" light had come on a couple of days before the trans issue popped up.
Thinking it might be a sensor issue, we disconnected the battery to reset the computer. The trans shifting normally for a short time after that, but then started acting up again. We did not turn off the engine before the problem recurred; the trans stopped shifting up while we were still driving it.
I have read that if the cat converter is clogged up it can trigger an error code in the computer that causes the trans to go into "limp home" mode to avoid overheating and potentially cause damage to the trans. I've also heard that a clogged converter can cause the engine to lose so much power that the trans can't physically shift up. Of course, at 230K it's quite possible the trans is going out, but I'd like to make sure that's really the problem before sinking a ton of money into a rebuild I might not need.
Can a cat converter cause trans problems like this, and if so, how much should I expect to spend to get the emission system replaced? Will an aftermarket cat work? I've priced the the OEM parts and they're very expensive. We live in the midwest so we don't have to worry about passing any stringent emissions tests.