Service Engine Soon Light problem

My 96 Firebird formula has just had a complete change of the head gasket assembly and
a new catalytic converter. However, the "Service Engine Soon" light keeps coming on
even after the mechanic reset it, saying he tested the sensor and found nothing wrong
with it. When he reset the sensor and tested the engine, it was operating perfectly at all
the GM factory settings. Any possible solutions? I need to resolve this to pass a smog
test.