About a month ago, my check engine light came on. The code was P0420 - Catalyst efficiency below threshold. I cleared the code and continued driving. I wasn't going to replace anything without some way to confirm the problem was fixed. The check engine light remained off for a full month.

Earlier this week, the check engine light turned on again. This time, the code was P2195 - O2S1 stuck lean. So I used my code reader (Actron 9185) to graph the voltage of the sensor over time. While idling or driving at constant speed, the voltage reads about 3.4v with small fluctuations. When I am parked and step on the gas, the voltage drops and then the car clearly adjusts the A/F ratio and the voltage rises back to about 3.4v. When I take my foot off the gas, the voltage rises and then quickly falls back to 3.4v. From what I have read, this all sounds like a normal A/F sensor.

Now if I am on the highway going 70 mph and take my foot off the gas, then the voltage spikes all the way up to 4.7v and remains there until I either accelerate or reach 25 mph or so. Is this normal? Does anyone know exactly what conditions will set off the P2195 code? Its clearly not stuck lean although it may be biased on the lean side.

Thanks for any help.

 

Air fuel Sensors must be checked regularly for the smooth running of our vehicle
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When you let off the gas, the motor is not getting any fuel at all if you are still in gear (manual only). It would be normal to get a high reading during this time. My wideband goes maximum lean when I let off while in gear. When you get to a low speed, the car naturally goes into anti-stall mode based off several parameters and RPMs will stay near 900-1000 until a full stop. They'll then drop to the proper 600-700 RPM idle range. This (from what I can figure) is to allow for extra vacuum load imposed by tight cornering with the steering and heavy braking situations so that the car won't bog or stall when doing these common traffic maneuvers.

How many miles are on your car and are there any mods to the engine? If the P0420 code tripped without mods, your primary O2 is most likely the culprit because it is giving the ECU incorrect voltages for AFR, thus creating either too rich or in your case, a too lean CEL. The secondary O2 is just a "tattle-tale" sensor if you will. It won't cause the engine to run differently if it is tripped, but if it is working properly and displays the P0420 code without mods, then your primary is out of whack and is causing improper AF mixture that the secondary will detect. Best of luck, I'll stay tuned.

 

rangerryda,

This is a 2006 tC without mods and 85000 miles on it. I'm caught between believing that it is the catalytic converter and believing it is the A/F sensor. I'm not sure how I could differentiate between these two failures but the computer seems to think that it can.

Last night, I noticed that the secondary O2 sensor reads approximately 0 while decelerating and about 0.7 while my foot is on the gas.

Is this clearly a bad A/F sensor or are there additional tests that I could run that would help me?

 

The secondary doesn't vary much in voltage. It's known as a narrow band sensor. This is normal. It is not the sensor detecting the lean condition, merely just the effectiveness of the catalytic converters. The fact that the lean condition was thrown as one of your CEL's leads me to lean towards the upstream/primary O2 sensor (located on the exhaust manifold under hood). I had the same exact codes thrown for my 2001 BMW and the culprit was the primary O2 sensors (2 on that vehicle). Estimated life on an O2 sensor is roughly 90-100k miles. If you are at 85k, then it wouldn't surprise me if it is faulty. That's about the time it started on the BMW.

As far as additional tests, you can smell your exhaust (sounds funny). If it smells more like a '60s muscle car than a new clean emissions car or you smell eggs (no joke), then the catalytic converter could be at fault but it's improbable. They are a very simple design and when unmodified, should operate perfectly until they rust from the outside in. Visually inspect the catalytic converter on the exhaust manifold (rust coloring is normal) and if all is well, move on to the sensor. Hope this helps.

 

rangerryda,

Thank you for the suggestions. You (and the computer) were correct. I replaced the primary O2 sensor and after driving 50 or so miles, the car passed all of its built in diagnostics. That is much further than it got all last week.

The graph for the primary O2 sensor looks just as it did before the replacement. However the secondary O2 sensor now reads things a little differently. Previously it read 0.7v while my foot was on the gas and nearly 0 when my foot was off the gas. It never measured anything in between. Now the secondary sensor readings are much more smooth. They still bounce between 0.7 and 0 but in a much more gradual way .

So now I'm wondering what went wrong. The sensor is covered in a whitish coating on one side. It almost looks like it was dipped in white paint. Is this normal?

Thanks,

Brandon