E520 air sensor shadetree fix

I have a e520 desktop.Put in refurbished motherboard.Old one was dead.Got error message air flow sensor not detected.I read all the post here.Some have a cable next to fan plug for sensor,my new and old board has no connecter there.Read flow sensor is on I/O board so bought new board and ribbon cable,same thing.Then I was told there is a sensor in the fan so I replaced with new dell fan 4 wire,which is what old one was and still same problem.If I send board back will lose some money and weeks of time.I'm wondering about this:

Since new fan doesn't speed up but does run and the heat sink is like 4" tall could I just add a second fan to rear of heat sink and plug into one of the extra power supply outlets and be safe?