O2 Sensors reading RICH, fuel trims adding MORE fuel.

[APRODS123]

Hello fellow fiesta owners!

I have an issue with my 2015 Ford Fiesta 1.6L where the car has thrown codes such as P0420 cat inefficient, P2096 post cat too lean, and P219A AFR imbalance.

The readings from the OBD2 data are very strange. The front wide-band O2 sensor is reading around -0.5mA(rich) while idling or driving and cuts to full lean under deceleration. The rear O2 sensor is reading the same, pegged at 0.9V (rich) while idling or driving and cuts to full lean under deceleration. Both O2 sensors have been replaced and data has not changed. The car is running long term fuel trims such as +20 LTFT adding MORE fuel to the fire. The car thinks it is running lean based off of the P2096 but it is indeed not, confirmed by a rich smell from the exhaust. I have so far changed both O2 sensors, changed the Purge Solenoid and cleaned the Throttle body as Mitchell's top fixes for this code and nothing has changed. I work at a shop and had a company diagnostic specialist come out to try and figure it out and we have not gotten anywhere. I believe the rich condition is failing the cat due to the excess fuel in the mixture and not due to the cat actually being bad. Oddly enough, LAMBDA is reading around perfect.

Any ideas? I'm leaning towards a computer issue. Thank you!

 

[Handy Andy]

Looks like an Occams Razor moment...

The PCM tells you you have a "Rich condition" yet the O2's show nearly perfect.

So either an open wire in the sensor wiring or the "Heater" that is used to make the O2 sensor effective in cold weather and helps the warm up process - if that wiring is bad or shorted, there goes the ballgame - so check the voltages to the O2 and make sure the return wires (the ones that GROUND) are not open - any of them (I know you show voltages as 0.2 and 0.9 in respect to the ... what? Only tested return wire - at the sensor? Did you trace back to the ECU and verify that reading? You may need to verify that the O2 sensor readings are not hindered at the PCM/ECU)

There is also the issue of the PCV valve - if that is plugged, this affects mixture as in too low in O2 to start with, affecting vacuum - idle quality and foul air (crankcase breather) - which will affect mixture.(by it noticing the Throttle Body plate angle is greater - thinking; it needs more fuel to the air even though the idle circuit seems ok...Again, this may be something simple but if you do not consider the Idle air "leaks" that are standard with the system, the Throttle plate opening will need to be greater than it was planned to trim to, because it's thinking the "leaked air" budget has not changed, note the air snorkel bypass, EVAP canister purge and the PCV, these vents all are BELOW the throttle plate and it's considered the Idle air in the air quality budget when it's at rest, and so it looks to a table to find something to trim to but then it sees the throttle trim angle to be different - but it's not in motion nor are the TCM/Manual or Brake and Speed sensors show movement - so even though stable - trim is not. The P2096 DTC is a generic result of the "All I can think of because everything else seems ok" moment.

The cat can also suffer damage so it has lost efficiency and so the O2 - it's not seeing any conversion process - even though you show it as a measured result. If the Exhaust Manifold has a leak - this can introduce a secondary effect of dilution and since these motors often run high vacuum - that air, getting sucked in, can cause a "lean condition" even if it's (the seal leak is) not there. So check the manifold and exhaust - make sure those joints are sealed - any changes or mods to the exhaust system - too much free (air and larger pipes) and not enough backpressure - can make the system think it's lean because it's trying to add more fuel to the injected air that shows up, it thinks that it shouldn't be there in the first place. (However lame this may seem to be in the process of elimination, the Catalyst can fail, causing this condition due to a cracked casing or a crushed honeycomb whereas the media that does the conversion - is simply not enough to change the outcome and output - so both O2's seem ok, but the Leading one (Bank 0) is calling the shots while the follower is also changing but at a rate that is too close to the leading ones rate of change - again setting this code because it cannot isolate any other condition - so this may be a trim issue with air quality below the throttles' plate and where the throttle angle rests at to find Idle speed stability - related to the Catalysts inability to convert the HCO2 to CO2 and water)

 

[APRODS123]

Thank you for your response!

I have fixed this issue and my findings have been interesting. Read along if you are curious on how the emission monitors work on this car.
The vehicle was using a downstream Oxygen Sensor Spacer with a Mini Cat to try to pass emissions testing. This was the entire issue.

Although the vehicle no longer set P0420 or any codes other than the occasional pending P219A when the spacer was installed, it also made it so the O2, H02S, and CAT monitors NEVER became ready, no matter how many hundreds of drive cycles. Without any codes, no one could figure out why the monitors would not set. One thing I noticed was although the readings from the downstream O2 with the spacer installed seemed normal enough to pass emissions and responded to deceleration, I later found it never gave the PCM enough accurate data to pass the monitors under any circumstance. So, normal enough data to not set any codes, but not ENOUGH data to pass the emissions monitors.
Useful knowledge; If anyone is trying to use an O2 sensor spacer with a 2015 Ford Fiesta SE 1.6L w/ wideband upstream O2, it absolutely will not work to pass emissions. The car is too smart.

Heres where it gets interesting:
In response to the vehicle running rich AND also adding more fuel, the car was actually doing exactly what it was designed to do in this scenario.
As part of the emissions readiness monitor testing that the vehicle conducts, the car needs to see the rear O2 respond to the front O2. If the car does not see an accurate enough response from the rear O2, it will FORCE more fuel in attempts to get a response from the rear O2. This explains why it was running rich AND adding fuel(LTFT). It was adding additional fuel to run rich by design in effort to get an accurate response from the O2. Because the O2 wasn't actually bad, it never set a code. The car gradually ran more fuel into the system until it reached its max limit of around +20LTFT (causing P219A) or -0.5mA from the upstream sensor. Because the sensor did not respond with a pass OR fail, the car became stuck in a perpetual state of richness, which explains why the exhaust smelled rich, the data was rich, yet the car ran perfectly and LAMBDA was perfect. The PCM was in control the whole time and knew what it was doing.

The spacer was removed to verify the failure of the cat and popped a pending P2096 on the first drive, as the PCM was adjusting to the rear O2 now responding as it should and gradually adjusted its own LTFT to a normal enough level to no longer set or post the P2096. This code was never stored and we will call it a "confusion code" as the PCM was learning the presence of a functional spacer-less rear O2 and adjusting the data likewise. After 3 drive cycles, all readiness monitors passed and I was left with a stored P0420 and the pending P2096 self-cleared, verifying the cat needed to be replaced. After the monitors passed, data was normal, the front O2 read near 0mA as it should, and LTFT was near 0.

After replacing the catalytic converter and clearing the code, it took another 3 drive cycles for all monitors to pass and the "long term memory" permanent Ford P0420 was self-cleared. The car is now inspection ready.

Thanks for the assistance!

 

[Handy Andy]

AH HA! Eureka! Said Archimedes - as he ran his Fiesta thru the Car Wash!

Know the size of the vehicle and the Owner of the establishment washing said car - he developed the Geometric principles of how to get your Horse and Carriage cleaner using less water.

ON a lighter note - WHEW. You certainly taught us a lot!

To know that the Cycles were never able to complete makes this a very interesting read on how to defeat an emissions test check that can (when it passes) then reduce the owners desired performance curve when the system finally develops the correct trim and emission profile - it was never allowed to finish it.

IT can explain why some times when you get a vehicle back from a dealership - it drives like a brand new car - acceleration and performance - all the like, making it seemly or remembering what it was like to drive it when you first bought the car - then one day, you start it up - it's sluggish, no pep and stinks and gets you nowhere - acting like it did just before the problems that made you drag it to the shop in the first place.

SIGH - that's the drive cycle blues - many would rather make the car act like it's first time out when the accelerator worked one way and you can drive like "this" and so it operated like "That" and when you think about it - it can shorten the life of the engine and car quite fast - that is supposed to be a type of break-in function that makes sure the car survives until the last payments are made. Whether that's true of not - the drive cycles are designed to test emissions - you feel it as the fun the car is to drive...

Wow!

Glad you found out - hopefully it won't make the rest of us think about doing something like that and just "tape over" the Check engine light.