testing dpfe circuit would have to be done while fault is evident.if it is an intermittent and when testing it will show good.I would clear code and see if it or how long it takes to show again.....turn on the light.if it is a hard fault it should trigger light right away.you stated that it has a random misfire......how often?under load?engine hot?any other codes?ford has a misfire counter built into mode 6 where you can see if there is any misfires in any cylinder seen by pcm but need a scan tool capable of accessing mode 6.this is very valuable but would have to view while it is actually misfiring.intermittents are the hardest to pinpoint and test.