2001 BMW X5 O2 OXYGEN SENSOR PERFORMANCE

I RECENTLY REPLACED ALL 4 OXYGEN SENSORS BUT THE TWO IN THE BACK WERE TOO EXPENSIVE, SO I USED THE ONE THAT WERE MADE FOR THE (6 CYLINDER ENGINE INSTEAD. IT IS A BIG PROBLEM?
BECAUSE MY SCANNER STRIPPING ABOUT (HEATER CATALYST NOT READY. (P1158/P1160)[img:f9fa8949f0]http://www.2carpros.com/forum/automotive_pictures/504541_WBAFB335X1LH27059_9_1.jpg[/img:f9fa8949f0]