Can a cat converter cause transmission problems?

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.

Don’t replace parts blindly.
Go to a dealer and get the error codes read. Post them here.

"we’ve been told it could do more harm than good replacing the fluid after 100K"
That’s an old myth.

Can it accelerate strongly when in 2nd gear?
That would rule out a plugged converter.

If it hasn’t been changed in that long, change it, and then change it again in 1000 miles or so. It may require a third change to clear out some of the muck that’s built up. Don’t believe the myth - I don’t anymore.

Every time I have been to a mechanic that can figure something out - catalytic conv.

We’re going to get the codes read when the dealer opens tomorrow. Sounds like changing the fluid might be the first step to see if that helps. Is it worthwhile to pay more for one of those transmission flushes where they circulate the fluid in and out several times, or is just draining the case and filling it up again good enough?

You can have them do that “flush” thing, but it’s really just a fluid exchange. That would be fine, although more expensive than a drain/fill. It also changes more of the fluid.

Regardless of which way you go, check the difference between the old brown nasty stuff coming out, and the nice pink, clean stuff going in.


Error codes dont mean a damn thing if you cant/dont diagnose them

Apostrophes don’t mean a damn thing if you can’t/don’t use them. (periods too)


“We also have never replaced the fluid because we’ve been told it could do more harm than good replacing the fluid after 100K.”

Sounds like the transmission is on the way out due to never having changed the fluid.

What you were told per the comment above is total bunk.

1 Like