Conditions required to set code are:

No VSS DTCs set.
No CKP, CMP, ECT, MAF, MAP or TP sensor DTCs set.
Engine speed 450-5800 RPM.
System voltage 9-16 volts.
ECT 21-248°F (-6-120°C).
Throttle steady.
PCM detects crankshaft RPM variation, indicating a misfire which could cause catalytic convertor damage or high emissions.