I own a Corolla (year 2000 model). I had P0171 fault few months ago. After internet search and discussion with my mechanic, I added one bottle of fuel injector cleaner available in Walmart. The check engine light (CEL) disappeared very soon. It however came back after few days. I repeated the same few times after each fueling. I also started adding mid-grade gas instead of regular grade. Further, one of my friends told me that I should run the car on highway after each time I add injector cleaner, to force the deposition to dissove faster. I was mainly using the city roads as my work place was