'02 with P0171 code getting better mileage?

You might have one or more of the following problems:


Air induction system

Injector blockage

Mass air flow meter

Engine coolant temperature sensor

Fuel pressure

Gas leakage in exhaust system

Open or short in A/F sensor (bank 1, sensor 1) circuit

A/F sensor (bank 1, sensor 1)

A/F sensor heater (bank 1, sensor 1)

PCV valve and hose

PCV hose connection

ECM

When the engine is running so lean that you actually see an mpg improvement, this may mean that smog emissions are up, with more NOx being formed. Also engine longevity may be impacted when operating under higher combustion chamber temperatures.​

Regarding the air induction system, check for a loose oil dipstick or oil filler cap. If not loose, inspect the rubber seals on those two parts.​

 

Check if your gas cap is on tight. Since it happened after you refueled...I always undo the last thing I did, unless I liked the new MPG.. which I do.

 

i have same prob, same code.. and better mpg also..

 

Anyone manage to resolve this? I've been seeing it for a few months now on my 2001 Prius, but otherwise no other symptoms or issues.

 

LOL you have an issue

• The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filter may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
• There could be a vacuum leak downstream of the MAF sensor.

ossible solutions include:

• In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
• Inspect all vacuum and PCV hoses, replace if necessary
• Check for a dirty fuel filter and proper Fuel Pressure

 

Most likely cause is bad MAF sensor, common on older 1nz engines. Try a new one.

 

I had these symptoms too. I had an oil change done and a new air filter on my 2008 Prius, which had been operating fine. Within 20 km the engine light came on, caused by a P0171 code. The car seemed to be going fine, and fuel consumption had improved by about 5%. If the code was reset, it re-appeared in 10-20 km. Of course, I was concerned about the engine light being on, so I took the car to a mechanic They checked for vacuum leaks (including simple things like the oil filler cap seal), cleaned the MAF sensor, replaced the spark plugs (which were due) and cleaned and reset the throttle body. The engine light is now off and the fuel consumption has improved by another 5%.

I am 99% convinced that the only thing wrong was that the old air filter was quite dirty, and the new air filter let a bunch more air into the engine and the difference was so great that the system perceived it as a problem. So I think it was resetting the throttle body that fixed it, which I could probably have done myself it I had understood it a bit more! It may even have come right by itself given enough time (though 100 km wasn't enough).

Interestingly, I haven't seen 'throttle body needs to be reset/calibrated/relearnt' in the lists of possible reasons for a P0171 code.

 

Most probably because for a trained mechanic it goes without saying. For others, they don't know what they don't know.