I have owned my 2009 FEH since new and have 214k miles. A couple of months ago I noticed the gas mileage was slowly dropping. I changed the spark plugs (had 100k miles on them), changed the air filter, checked tire pressure and even started driving slower and in more EV mode. Nothing helped, the mileage kept dropping. I was ready to take it to a mechanic when I got a check engine light on the dash. With Forscan I found I had a P0128 code. (engine coolant temperature is below the thermostat regulating temperature) I checked the coolant temperature and found it to be 165 degrees, about 35-45 degrees too cool. I had the coolant thermostat replaced and now I'm back to my original gas mileage. (about 5 mpg better)

Two lessons learned here: the obvious one now: a cooler engine make the gas/air mixture run richer to compensate. The other lesson is sometimes the check engine light takes a long time to come on.

I know these lessons will be obvious to the many folks on this board who know way more about vehicles than I do, but hopefully this post will help others (like me) who aren't so knowledgeable.

 

Coolant temperature factors into:

1) fuel mixture. When coolant is cold, more rich mixture.
2) Hybrid specific: Engine will run more to bring the coolant to operating condition to avoid #1. More running engine when it otherwise wouldn't = less mpg.