Fuel Trim DTC's - Looking at the Big Picture

Louis Diagnostician California Posted   Latest  
Case Study
2006 Ford Explorer XLS 4.0L (E) 5-spd (5R55S) — 1FMEU63E66UA99215
P2195 — O2 Sensor Signal Biased/Stuck Lean Bank 1 Sensor 1
Engine Noise On Cold Start

Hello everyone, 

I'm sharing an experience here that proved to be somewhat of an ass-kicker and wanted to illustrate what we found after spending too much time focused on the obvious (DTC) and not on what should have been focused on initially. When you have an experience like, we believe that it's always good to look back and review what was learned which, in our case, always helps us grow as technicians.

The above vehicle with 132k miles arrived with the MIL illuminated. A scan was performed and the following was recorded:

  • DTC P2195 - O2 Sensor Signal Biased/Stuck Lean B1S1

We looked at the usual suspects during our initial inspection which includes vacuum leaks or exhaust leaks on the problem side. Initially, we did discover vacuum leaks between the upper and lower plenum. Anyone familiar with this engine has likely seen this condition in the past. At this point, we felt that we had a slam dunk and sold the repair. Post repair, you guessed it, we had the same DTC return. 

Looking at the Ford OE Service information website, one is presented with the following potential causes for this DTC:

Description: A heated oxygen sensor (HO2S) indicating lean at the end of a test is trying to correct for an over-rich condition. The test fails when the fuel control system no longer detects switching for a calibrated amount of time.

(I'm having a hard time understanding the statement above about the "trying to correct for an over-rich condition") Can anyone share their interpretation?

Possible System Causes


  • Short to VPWR in the harness or HO2S Water in the harness connector
  • Open/shorted HO2S circuit
  • Corrosion or poor mating terminals and wiring
  • Damaged HO2S
  • Damaged powertrain control module (PCM)

Fuel System: 

  • Excessive fuel pressure
  • Leaking/contaminated fuel injectors
  • Leaking fuel pressure regulator
  • Low fuel pressure or running out of fuel Vapor recovery system

Induction System: 

  • Air leaks after the mass air flow (MAF) sensor
  • Vacuum leaks
  • Positive crankcase ventilation (PCV) system is leaking or the valve is stuck open
  • Improperly seated engine oil dipstick
  • EGR System:
  • Leaking gasket
  • Stuck EGR valve
  • Leaking diaphragm or EVR

Base Engine: 

  • Oil overfill
  • Camshaft timing
  • Cylinder compression
  • Exhaust leaks before or near the HO2S(s)

During the second round of analysis we gather the following:

I'm interested in hearing what others see in our data and explain what they see.

I appreciate the feedback and plan to share my conclusion soon following the discussion I hope this generates.

Matt Diagnostician

Were you able to reproduce the code setting and view data before, "during" and after? What were the post-cat O2's doing, as well as STFT? Thanks, Matt.

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Louis Diagnostician

Hi Matt, Thanks for the questions. Unfortunately we were unable to secure approval to fix what we determined to be the ultimate cause. As for the data you’re requesting, I failed to capture/record any of that info. What we finally concluded as the cause was the VE variance bank to bank. We feel (based on the compression waveform analysis) that this was the cause of the fuel trim split. The

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Matt Diagnostician

The "actual" vs "desired", or Error PIDS for cam timing would have been interesting to see. The trim split wasn't enough, for me, to jump on that right away, and part of the reason I was interested in STFT. Great find!!!!

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Brady Owner/Technician

According to your pictures there the valve timing is quite a bit different from bank to bank, that'd be my first concern.

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Albin Diagnostician

Interesting problem to say the least. Reading the code set criteria, I can see how following that information could lead you down a rabbit hole. Any time I'm confronted with a problem like this, I want to use; RPM, TP, long and short term on both banks, MAF & Load on my scan data. Capture on a test drive from idle, WOT & cruise. With the scan data you posted, I don't see any evidence

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