P0171 16555 Fuel trim too lean

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Per_Norrgren
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Posts: 2
Joined: Wed Oct 10, 2018 12:36 pm
Drives: 9N 2002 1.2 64Hp
Location: Loule, Portugal

P0171 16555 Fuel trim too lean

Post by Per_Norrgren »

Hi,
hope someone can help.

Polo 2002 1.2 Petrol 47/KW
Vin WVWZZZ9NZ3Y057956
Var: SC AZQ X01 ; VER: SGFM5 2T037 1N1 L0GE
Engine: 9N1 0D4

I have seen some great posts on this issue but I seem to have a slightly different take on it.

I am using a AUTEL MaxiDAS DS808 diagnostic tool to view the real time ECU information.

I bought the car 4 years ago. some month after purchase the engine light came on and the car also did not pass the CO2 test at the next MoT.
The erroe codes indicated that both the lambda sensors, the Air sensor on the manifold and the cat needed changing.
So all units changed (including the manifold and petrol filter for good measure) and all was good for a while.
some time later the car started intermittenly log the 16555 Fuel trim too lean code and the periods between logging can be 10 km to 100 km.

using the diag tool I get the following when viewing live ECU data:

O2 sensor 1: Idle 1.25V, 2400RPM 1.37v and it is quite steady. only dipping to 0 for a second when taking the foot off the accelerator
O2 sensor 2: Idle 0.74V, 2400RPM 0.7v and it is quite steady. only dipping to 0 for a second when taking the foot off the accelerator
Result Delta Lambda B1: B1-S1 OK
Result Lambda Availability: B1-S2 OK
Intake Manifold Pressure (G71): Idle 295 mbar 2400RPM 244 mbar (removing a vacuum hose, causes very rough idle and there is strong vacuum when you put the finger on the hole)

Question1: Should not the O2 sensor 1 be alternating between low and high value?
Question2: Should not the intake Manifold Pressure be 500-600 mbar on idle?

Any thoughts / insight / help greatly appreciated

thanks and regards
Per Norrgren
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