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Old 12-26-2018, 11:09 AM   #1
Drives: Yaris 2001 VVTI 1.0l
Join Date: Sep 2015
Location: France
Posts: 6
P0171 Error Code

Hello everybody,

I have a problem with my 2001 Yaris (Made in Japan). When i'm running on the highway the CEL came ON with a P0171 code. I have made a lot of research and i + the toyota dealer haven't found the answer. The code appears only on the highway.

I have some symptoms -> Increasing of fuel consumption, sometimes engine hesitation with power loss.

I have cleaned the MAF sensor, Replaced the lambda sensor after the cat (Toyota said that the first one is ok). I have checked for leaks but i have not disassemble the intake manifold.

I have purchased a Delphi OBD reader and i have checked the values between my 2 yaris. The Engine load is weird around 30% at idle but the MAF values are identical on the 2 cars. Long term fuel trim is around 20%. At 80kmh the first O2 sensor oscillate between 0,8 and 0,2v - The second is around 0,7v.

I have checked my spark plugs which are pretty white, so i'm assuming that the engine is really running lean. but if i'm not wrong, this is not coherent with the lambda values. The first cylinder and coil is really dirty around the spark plug like if the spark plug was leaking ?

This car made a lot of highway (70 000km at 130kmh) and overheated once.
The car has 165 000km on it.

So now i am asking if :
- Bad Fuel Pump ?
- Air Leak ?
- TPS sensor due to the weir engine load ?

I don't really know what i should change or if i stop now, considering that toyota charge me 500 bucks for the diagnostic and O2 sensor with no results.

Any advice will be well welcome

Thank you !
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Old 12-28-2018, 04:40 PM   #2
Drives: 2018 Yaris SE 5MT
Join Date: Aug 2017
Location: Kincardine, ON, Canada
Posts: 273
Sounds like you're getting too much air, so my guess is either vacuum leak - air ingress via a leak around the intake or via emissions connection. Or your MAF is under-reading how much air is coming past. Cleaning is a good first step, but could be a faulty MAF.

If your 2 Yaris' are of similar years, can you swap the MAF from the known good Yaris into the one that's giving you trouble, clear all the codes, and see if it solves the problem? Or if you drive the other one with the suspect MAF see if you get the P0171 code on the other Yaris..

Let us know how you make out.

-- Adam
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Old 12-30-2018, 01:50 PM   #3
Drives: 2000 SCP10, 2001 NCP13
Join Date: Jul 2016
Location: CZ
Posts: 53
Check the live LTFT values. Running rich (--> large negative corrections) at idle and low load & lean (--> large positive corrections) at high load/WOT = dirty MAF sensor; the opposite is usually a vacuum leak (less throttle = more vacuum = more unmetered air sucked in).
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Old 12-31-2018, 02:07 PM   #4
Drives: Yaris 2001 VVTI 1.0l
Join Date: Sep 2015
Location: France
Posts: 6
Hello everybody !

I have found a lot of parts in a car breaker. I have changed :
- The MAF sensor
- The PCV Valve
- The entire throttle body with the TPS sensor and the idle air control valve
- The gasket around the throttle body (intake + Air box)
- All the hoses around the throttle body

I have purchased all those parts for 60. I think it's a bargain !

I have seen that the value of the old IAT sensor where way off (Like 10/15C more)

Now, with the new parts, the values on the car seems quite good. The long term fuel trim is constant at 1%. The car idle nicely at 600rpm and it seems that i have retrieved some power.

I will wait to do a long trip on the highway which was the condition that triggered the code everytime. I will let you know if it's solved !

Thank you everyone for your responses !
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Old 12-31-2018, 02:26 PM   #5
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Drives: 2008 Yaris HB 5-Speed
Join Date: Mar 2012
Location: Ashland, VA
Posts: 356
Way to go!
I'm a hermit named Lee
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Old 01-04-2019, 06:33 PM   #6
Drives: Yaris 2001 VVTI 1.0l
Join Date: Sep 2015
Location: France
Posts: 6
After 600km with 400km on the highway at a speed of 130kph i have no Check Engine Light ! The car a regain some power, it's revving higher on the starter, and i have no engine hesitation.

So i assumed that the problem is fixed !
Thanks for all !
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