What does PO171 mean for 2000 Toyota Echo.

That 171 OBD code is for a "lean condition" (not enough fuel being added).
This is a common false-ish alarm on Toyota Echos and results in the check engine light coming on and poor engine performance. It happened to me and a a few friends as well as many on the net (Google 171 and echo)
In my case, I had poor fuel economy and poor power performance along with the check engine light. I took the car to the Toyota dealer and they charged me $90 to stick on a OBD reader to see code 171. They said the MAF needed to be replace for $130!!!
I immediately checked online and found that the 171 problem was common to many post 2000 Toyota's and was due to dirt accumulation of the heater wires in a MAF (my guess is that the EGR gasses somehow contaminate it). The solution was to simply clean the MAF heater wires by spraying brake cleaner on them (To remove MAF you just need to remove an electrical connector and 2 Philip screws) The MAF is on the air tube in front of the air filter. To clean, spray in the "straw-like tube" from the bottom of the tube. (You can see the heater wires at the base of the tube with a flash light). Don't mix these two wires up with the temp sensor amber glob (which is very obvious to see).