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Hi,

I have a 2010 Mazda 3 and I have been trying to diagnose engine code P0171 for quite some time now, unfortunately without any luck.

What I have tried so far:
1. I have checked for vacuum leaks using carburettor cleaner and there doesn't appear to be any
2. I have replaced the MAF sensor
3. I have run a tank of Techron Fuel system cleaner in an attempt to clean the fuel injectors

When I drive the car and read live data with OBDII (Ancel AD310):
1. Oxygen sensor reading remains between 0.1-0.9V, so I believe the oxygen sensors are also ok?
2. Short term fuel trim stays in the single digit (+/-) range
3. Long term fuel trim is constant at 20.3%, however when I accelerate it reduces instantaneously to either 9.4% or 7.2% (It only ever seems to read those 3 values)

Is it worth trying to replace the oxygen sensors? Could the engine be running lean to correct the sensor voltage? (I'm not sure if that's how it works).

And why does long term trim only ever read 3 distinct values? I was under the impression it was more of an average that changes gradually. Is this down to the Ancel 310 or could I have an electronic issue?

Finally, my next option is to replace the PCV valve but I think it's a little over my head (and toolbox!) and a mechanic has quoted me $400. So I'd like to explore everything else before I go there.

Thanks in advance for your help!
 
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