I had that Exhaust Temp Sensor light on in my GTR for a while, not long after I had new turbos fitted, the car started running like a dog and the Cat was steaming! (Cat and dog reference pure coincidence! )
So initially I thought the Cat internals had melted...and well...had it removed.
Car still ran like a dog and would only run on the smallest of throttle openings.
To cut a long story short, I stumbled across the problem when removing the Plastic cover which bridges the rocker covers on the RB26, Not long after having new turbos fitted I painted the rocker covers, and had removed this panel, when putting it back on I did not plug the ignitor back in properly (Mounted on the panel) hence the plug came loose and car ran like shit.
Put the plug back in and No problems to this day!........