We all have been trained to listen to TCAS over ATC. No confusion there. A while back, on a flight into IAH, we had a blip show up on our TCAS display at our altitude, converging. ATC had said nothing to this point. The captain (the flying pilot on this leg) and I kept an eye on the TCAS display, as well as outside, trying to find this traffic. A second or so later, we heard the "Traffic! Traffic!" alert, followed by "Descend! Descend!", and the VSI indicated it wanted 1,000 fpm down or more. The captain disconnected the autopilot and immediately started descending.

I got on the radio and told approach control that we were descending out of 9,000 for a TCAS RA. Approach immediately came back telling us that we needed to stop descending, we had traffic below us at 8,000 feet.

I responded, telling approach that we would continue to descend as long as our TCAS told us to, and that we were still descending.

The controller again insisted that we stop our descent. I again told him we were complying with our TCAS RA.

At about this point, the RA went away, and the blip that we previously saw on our TCAS display was suddenly indicating itself at 8,000 feet, not 9,000 feet like it initially showed.

We were at 8,500, and started climbing back to 9,000. I informed approach.

We were then given a phone number to call, and when we got on the ground, we explained this story to an approach control supervisor. When we got home, both the captain and I filled out ASAP reports, and nothing more has been done.

This concerns me because clearly the TCAS was wrong in this case. The controller was right. Having said that, we did exactly what we are supposed to do by obeying the TCAS before the controller.

It just scares me that by doing things by the book, we put ourselves in danger. Anybody else had something like this happen?

Is TCAS dependent on the Mode C functions of transponders? Those are very often wrong...