2003 prius showed hybrid system error

I bought a 2003 Toyota Prius with 130K mileage with the hybride system replaced at 100K but not the auxillary battery in September 2009. Before I buy it I took the car to a mechanic and he reported me that it's a nice car to buy, and he didn't find any problem. I was happy with it for last 3 months, but recently as far as winter starts I started getting less mileage (27-35 mpg), before that I used to get 40-45mpg. I got around 20-25 mpg in first 10 mins, and after that mileage go up. I was very much concerned about it and couldn't find any reason for that except heating. However, last week I had a doctors appoinment and on my way back home I stopped in a grocery store at around 7:30 pm, when I tried to start the car after grocery, I discovered two warning signs 1) A Red triangle and 2) A car with an exclaminatory sign inside, which refers take the car to toyota dealer immediately in the manual. I looked online for the possible reason and find a recommendation to check throttle plate stickiness, When I opened that part I saw the air filter was almost clogged, so I bought a new air filter and a throttle cleaner (as per the instruction), I changed the air filter and spray some cleaner, than started the egine, and all the warning signs disappered. After that I drove around 100 miles and the signs did n't appear agin but I'm still getting less mileage, although slightly increased for what I got in the erly winter, so far i got 38mpg. So, is there nything else I can do to improve mileage? Also, Is there any recommendation to do anything further to prevent such situation/

I'll appreciate any suggestions in this regard and also thanks to the forum as I was able to fix my problem while reading the threads.

 

You are right, I got the same feeling aslo. However, in last two days I got 40mpg, as the temp. was not that low.

 

Sure, I'll.