Front Upstream O2 Sensor Low Voltage

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Buick Ownership
1999 Buick Regal GSE
My front upstream O2 sensor that came with the car died, I think it was even factory. Came with the codes PO131 and PO171, and also worse MPG. With my somewhat fancy scan tool, I was able to see that both fuel trim levels were stuck on 1 number and never changed. I bought a Denso O2 sensor and it installed. Fuel trim levels looked fine but I got the PO131 and PO171 code again. Bought another Denso O2 sensor and got it installed because I throw away the other box because they said Denso's a good brand and didn't need to worry. This time, Short Trims were changing but the Long trim was initially stuck on 20 something but eventually revert to 0. I get the PO131 but not the PO171. The car runs great and it never has any stalls and wild revs but my MPG is still worse then normal. After 3+ days of driving and before a battery unplug, I get a whopping 12 MPG. I do have the famous regal problem which is the fuel level sensor (Ex: 2 gallons is 3/4 full), so maybe the MPG is calculated into that equation but I'm certain its not. The PCM used to give me a error code for the Rear O2 sensor but it seems just to forget it and not even care, no matter how many times the PCM resets, honestly that's how I feel about the rear O2 sensor too.

If your curious about the Denso I was (I guess still) using is the "DENSO 2344018".
 
This kind of twisted into a different forever Error.

I farely certain the 2nd Front Denso is working properly, so the PO131 very rarely pops up and goes away by itself. PO171 always stays like a leech. The car still runs fine and idles below 1K. Hot or Cold, it runs the same. I mainly drive on the highway and still get an AVG of 12 MPG.

After a lot of PO171 searching, the possible causes:
  • Simple Vacuum Leak.
  • Fuel Filter and/or Fuel Pump.
  • Fuel Injectors
  • Fuel Injector Pressure Regulator (FPR)
  • Vapor Canister Purge Valve / Solenoid
  • Positive Crankcase Ventilation Valve (PCV)
  • Bad Ground somewhere.
  • MAF Sensor
  • Front O2 Sensor
  • Intake Gaskets
  • Exhaust Gases
We have replaced the Coils, Spark Plugs, Spark Wires, PCV, Air Filter, Water Pump and Thermostat since I got the car. I would love to through parts at her but honestly I want to check the MAF sensor and the fuel trims levels better.
 
Grabbed my fathers OB2 Bluetooth dongle.

So the voltage for the Front O2 sensor seems fine as it jumps between 0.01 to 1 volt. Weirdly, my fuel trims would not change at all (Even with my fancier tool). My bank 1 trims are always STF at 20.31% and LTF at 16.41%. Steady 1500 RPM, no change. Slight punch near 3500 rpm, no change. It also said I was getting 12 MPG avg. Also, the rear 02 sensor does work, I guess it fixed it self and that's why the DTC never came up for that.

Old friend PO404 error code popped up, it comes up more often then the PO131. It runs and idles great, so I think this whole situation is just weird. I wish it was cut and dry like bad Water Pump.
 
Bit of an update. Replaced the EGR and attempted to replace the Vapor Canister Purge Valve. The ACDelco Purge Value (Part Number 2142246) make my car idle horribly and lacked any power. It made a constant clicking sound. Put on the old one, and it ran great. I had so many bad parts from ACDelco, that I feel insulted because everybody and there mom tells me "How can you go wrong with OEM parts?". The EGR didn't solve the lean code but it needed fixed anyways.

My current theory is that the rear exhaust gasket has failed and is pouring constant fumes on sensor, what a stupid problem. If I drive it slower then a grandma going to church, 12 MPG. If I drive it and the RPM always hits 3K RPM, 12 MPG. I believe that I'm actually getting 12 MPG but there's a chance it could be wrong. My sending unit is basically broken and so I can't count miles manually. Sure I could replace the fuel pump but its working fine and also nobody makes sending unit kits anymore. I feel salty because my car is sucking more fuel then a 572 rat rod chevy.
 
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