P2196 - how to tell which O2 sensor? (2010 Prius)

Both air/fuel and O2 are id'd in this Repair Manual excerpt:

 

That's a good illustration of why it's rarely useful to try to make decisions from the text "fortune cookies" shown for codes on scanners, especially generic ones, or generic code lists or internet searches.

If you turn to P2196 in the repair manual (more info), your questions get answered pretty well right on the first page:



... plus you get 24 more pages in that section telling you what can happen to set that code and what tests you can do to figure what the cause was in your car, and what to do about it.

 

Here's that:

 

Did the thread originator ever find the root cause of the code?? I have the exact same situation (replaced engine in a 2010 and got this code). Thanks for the response in advance!!

 

I see that Mendel posted the code details and troubleshooting steps from the repair manual in post #5. The best way for you to find what's causing the code in your car will be to follow those, just as it was the best way for the thread originator to find what was causing it in his car.

 

Hello hope you all doing great need your help I have been to mechanic and he hasn't been able to diagnose exactly the problem
I am getting these 2 codes
P0420 (catalyst system efficiency below threshol Bank 1)
Mec asked me to change fuel ratio sensor (one in engine compartment)
After changing that I start getting another code
P2196 (O2 sensor signal biased stuck rich bank 1 sensor 1)

Should I change the sensor at the bottom by exhaust ?

 

Just changed the engine too, and after 2 days testing the car just got this code. How did you solved it? I just cleared the code thinking the ECU needs more readings with the new engine and it's sensors to adjust.

 

Mendel attached the troubleshooting steps in post #5 here.

Between what caused the P2196 code for wsalopek, and what caused it for JCEze, and what caused it for Kashif1, and what's causing it for you, you'll benefit the most from finding out what's causing it for you. The troubleshooting steps should help you find that out.

 

Have you read it? The only step missing is to stand in one feet, jump 3 times to the right another 3 to left and then say Shazam! I have in common with jceze that we both changed engine and after that we got that DTC. I kept most sensors on replaced engine, but probably it is better to install those from the old one starting with the A/F ratio sensor. Fortunately, I am still keeping old sensors and parts in case something may be needed during the first testing days days. Hopefully jceze can confirm what worked to him. "JCEze was last seen: Dec 27, 2021" or maybe not

 

I've looked at it ... what I see is 25 pages of information about what the code is telling you, including 15½ pages of inspection steps to try to pin down what could have caused the ECM to see what it saw. The steps include things like:

• Forcing the actual mixture rich and lean to see how the sensor readings respond
• Checking the sensor reading during fuel-cut deceleration
• Checking the sensor readings at idle, at 2500 RPM, and at closed throttle
• Checking the sensor heater
• Checking for proper EGR valve operation
• Checking sensor wiring
• Checking for a dodgy ECM
• Checking for intake leaks

which seem a lot like the kinds of things that would need to be checked to pin down one of the numerous possible root causes of this code.

It's not a code I've personally had to troubleshoot before. If I ever do, I'm sure I will find this procedure tedious and the interpretation in some places to be puzzling. I might end up hoping for help from @mr_guy_mann or somebody. But I would probably have to dig in and start doing it. When there are so many ways a code can be caused, the odds aren't very good that what's causing it for me will be exactly what caused it for somebody else. (Even if the somebody else and I just did the same very invasive thing like an engine replacement—there are enough ways to leave something catawampus in such a project that it's still not that likely we have the same root cause, unless we managed to make the exact same mistake doing the work.)

 

I wanted to share my recent experience with the P2196 error code, hoping it helps someone save some money in the future!

1. Replaced the upstream O2 sensor with genuine Toyota parts—no luck (~$300).
2. Replaced the downstream O2 sensor with genuine Toyota parts—still no luck (~$300).
3. Replaced the MAF sensor—no luck (used a donor part).
4. Cleaned and tested the fuel injectors—no change.
5. Cleaned the EGR valve—still no improvement.
6. Replaced the entire catalytic converter and exhaust system with genuine Toyota parts—no luck ($2200).
7. Finally, replaced the fuel pump with genuine Toyota parts—Bingo! The error code disappeared immediately, didn't even need to drive 100 miles ($500).

Who would've thought a faulty fuel pump could trigger a completely unrelated error code P2196. The only reason I replaced the fuel pump was due to a casual suggestion from an experienced mechanic at Hybrid Pit. Big kudos to him!

Jack

 

Thanks for sharing! It is just unfortunate that ya had to pay almost current value of the car.

 

If the injectors are not getting enough pressure, or intermintant flow/pressure, the
injectors cannot produce a mist so the fuel won't burn fully so the O2 sensors
will report too rich of a mixture. Then the computer will then lean the mixture, and things
will get worst.
Mercedes had this issue on some of cars in the late 80's....

That's why it's always good to do diagnoises to find out WHY they reporting what they are.

 

Well, I just replaced the a/f ratio sensor with the one from the exploded original engine and no more dtc, just 15 minutes job with the right socket for oxygen sensors.. It is just strange the sensors looks different since the original sensor does not required special tool. The one from the new engine had a part number stamped, and when googled is indicated to be for a prius 2011 too. Maybe the computer is just used to the usual signal range from the original sensor.

 

In my case, I did what was the most logical reason for the code. But I was ready to do the the 15 pages troubleshooting procedure and the rain dance if the replacement didn't work.

 

Sometimes, you just get lucky!

 

I know this thread is a couple years old, but I had the exact same problem and wanted to share that I resolved it using AI assistance.

My 2010 Prius had failed head gasket (verified by pressurizing coolant system and inspecting each cylinder with an Endoscope). Swapped in JDM engine, engine ran great, enjoyed 15% increase in gas mileage but...2 days later P2196 popped up. Assumed at first it was ECM getting used to new stuff so cleared the code, but 2 days later it came back. Left it on and drove it, monitoring with OBDLink, from what I could see monitoring O2 sensors and Cat temps using graphing mode it all looked normal and healthy (and gas mileage went from 40-42mpg to 47-50mpg. NOTE: mechanic accidentally allowed aux. battery to go dead while accessing ECM which damaged program so ECM had to be re-flashed adding a layer of uncertainty.

From internet research I suspected the A/F sensor, but wanted to try verifying it. Having a dozen other irons in the fire I didn't have time to fully wrap my head around the Toyota Repair Manual process so I tried describing issue to AI. It took several iterations to arm wrestle AI into revealing the root cause.

As recommended by AI, I used OBDLink to monitor 2 PIDs:
- "A/F Sensor Bank 1 Sensor 1" voltage and
- "Short Term FT B1S1" (short term fuel trim)%.

I created a dashboard displaying both a digital gauge and graphing gauge for both PIDs. AI also suggested I monitor LTFT (Long Term Fuel Trim), but that always read 0% and never changed. NOTE: For some reason OBDLink showed all of these PIDs twice, I monitored both versions of each both digitally and graphically, but determined both versions gave identical results.

First I drove 5 min to warm up to >180F (per OBDLink), then monitored the above PID's with engine at idle, then 2500rpm, then snapping throttle closed forcing ECU to cut fuel. AI interpreted the results during those 3 scenarios to arrive at conclusion - even though it was maintaining proper mixture, Japan uses a different O2 sensor in exhaust manifold (A/F) which is giving unexpected voltages causing it to "run blind". This conclusion was not too surprising since sensor looked different, it did not have the shield around the hex forcing you to use a special socket to remove and the sensor tip is fatter. There was corrosion on the body of the JDM sensotr so I could not read the numbers. I don't know whether to fully believe AI, but it also indicated Japan versions uses slightly different intake manifold and fuel injectors, even different cam profiles and higher compression??? Seems unnecessary for Japan (or US) to require so much change.

Initially the A/F held pretty steady at 3.3v at idle and 2500rpm and AI said this was ideal stoichiometric A/F mixture, but it peaked at 5v with throttle snaps which "...violates USDM ECU software parameters" causing ECU to interpret these short bursts of max voltage as "stuck on high voltage" (maximum enrichment). Oddly, STFT was running 6-7% like it was always trying to add more fuel even though I was getting great fuel mileage. At this point AI was positive that it was the JDM A/F sensor being calibrated slightly different than US spec sensors so I swapped in the sensor from the old engine.

After swapping in the US spec sensor the A/F voltage still held stable 3.3v with engine at idle and 2,500rpm (minor 0.1v fluctuations during throttle position changes), and stayed below 4.8v when snapping throttle closed. Per AI, STFT "fluctuates safely between +1.6 and +2.3%" @ idle and "hold safely between +3.9 and +4.7%" @ 2500rpm. "This rising trim rules out vacuum leaks and confirms JDM engine efficiency adaption."

AI inquired whether I had used the original or JDM intake mainfold, EGR system and fuel injectors. I responded that intake manifold and EGR system were from US engine (I had insisted mechanic use my original parts since they were like brand new, i had just cleaned them while addressing original misfire code that eventually led me to discover head gasket failure). I did not know for sure about the injectors. AI was thrilled that I had chosen to use the original manifold and EGR system and went on to say that the STFT readings indicate the ECM was likely working to correct for the JDM injector profile but it was well within normal healthy operation and safe to leave as-is.

At one point I took a break (another iron in the fire) and the AI conversation that led to the JDM A/F sensor as culprit disappeared so I had to start over. I must not have used the same prompts because AI did not arrive at the same conclusion as quickly (I eventually told AI about the previous conclusion to force it to reveal the secret a 2nd time - it sounded a bit miffed, but concurred with that conclusion). Unfortunately I lost the original conversation so I can't share the prompts I used.

After seeing repeated occurrences of P2196 after a JDM engine swap, and as vallesj and others discovered, swapping the A/F sensor eliminated P2196, I am convinced that the JDM A/F sensor is the key issue if P2196 appears after a JDM engine swap.

 

Huh. I've never really had an AI sound miffed yet. Usually it just congratulates me on my brilliant catch of the point it had been intending to eventually make....