95 Corolla ECM code 71

Looking for advice on where to go with this problem... any help appreciated.

Have a 95' Prizm (same as Corolla) 1.8L. A spark knock problem popped up suddenly. I was advised to have the EGR checked and get the injectors cleaned for carbon buildup.

While I was on the road to take it in for that, the check engine light came on. While doing the injectors, the service tech put it on the scanner and told me it was code 71 "EGR Temp. Sensor."

I just replaced the EGR valve yesterday, with a new AC/Delco unit. Had to replace the whole valve since the sensor is not available by itself. The ECM was reset and everything was fine for a couple of trips but now the code 71 is back.

The only thing I can see is that the EGR solenoid filter has some carbon. The old EGR valve was also pretty cruddy inside.

Any ideas what to check next?

Thanks!