Is there any easy way to identify which sensor has failed? I don't have an OBD2 reader, but I know that one of the O2 sensors has failed on my 2002 Sirion Rally IV as my local garage put it on a diagnostic and identified this. Just to complicate matters, they forgot to record which one it was, and now want to charge me to put it back on a diagnostic, which understandably, I didn't agree with as they were the ones who forgot to record which one it was...

Any help appreciated...