Air/Fuel Sensor Deterioration

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The guy I work with is trying to convince me to replace the air/fuel sensor on my Prius due to mileage. In his opinion (he is also tunes performance cars), older air/fuel sensors have a slower response time and therefore, a new sensor should help improve the computer's ability to make more accurate fuel corrections.

The car in question has 185K and I have not noticed a sharp decline in fuel economy or driveability.

Thoughts?
 
not aware of this being a significant issue.

Fact: most OBD-II systems with wideband upstream O2 sensors, their ECU has built-in tests to report both upstream and downstream O2 sensor issues, including "lazy" sensors (esp. upstream one which is wideband and heated also)....I had dealt with (incl. wifey's camry) upper sensor reaching the test "threshold" by ECU and had to be replaced to restore proper functionality.
 
Hook up a scanner and see what the fuel trims and sensor voltages look like.
 
Originally Posted by Quest
not aware of this being a significant issue. Fact: most OBD-II systems with wideband upstream O2 sensors, their ECU has built-in tests to report both upstream and downstream O2 sensor issues, including "lazy" sensors (esp. upstream one which is wideband and heated also)....I had dealt with (incl. wifey's camry) upper sensor reaching the test "threshold" by ECU and had to be replaced to restore proper functionality.

I think my friend's theory is that the "thresholds" are quite significantly, so even if the system does not trigger a fault, there is still room for improvement.

Originally Posted by mclasser
Hook up a scanner and see what the fuel trims and sensor voltages look like.

That won't tell you anything useful, at least not for this specific purpose.
 
Mine has 270k and works fine.
smile.gif


Watching it in Torque is fun; above 50 MPH when I coast the engine keeps running but at an 18:1 AFR instead of 14.7.

When the gas motor kicks on and off there's the tiniest surge in AFR but it settles and locks in very quickly.

This is of course the computer's opinion of what's going on. My 2nd gen prii are slightly "bumpy" kicking in and out, particularly when warming up, but apparently normal by design.
 
Originally Posted by mclasser
Hook up a scanner and see what the fuel trims and sensor voltages look like.


That would be assuming the MAF is reading correctly.

I would compare fuel trims before and after cleaning the MAF.
 
I've heard it's recommended to replace them at 100k, but you can easily go much further.

Replaced the oxygen sensor in my Caliber last year... no matter how I drove it got a constant 20-22mpg, calculated by hand. New sensor got me 25-27.
 
I replaced the original afr sensor in my 04 civic last year because the heater wasn't working. I drove it for almost a year with the bad sensor then changed it for a denso sensor. The car always ran the same and the mpg never changed. If we didn't have state inspections where I live I wouldn't have changed it.
 
Originally Posted by bdcardinal
I might have done it when the head gasket was replaced on the off chance coolant contaminated the sensor.

I forgot to mention that. Good point.

Apparently, BMW recommends replacing Oxygen Sensors every 120K or 150K depending on the vehicle. I have heard this before and I thought it was a rumor, but apparently it is true. Take a look at the snapshot from the maintenance manual below - it was taken from the manual of a 2015 BMW 3-series. Very interesting...


BMW.PNG
 
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Originally Posted by gathermewool
Originally Posted by mclasser
Hook up a scanner and see what the fuel trims and sensor voltages look like.


That would be assuming the MAF is reading correctly.

I would compare fuel trims before and after cleaning the MAF.

MAF was replaced 35k ago due to a failure.
 
Originally Posted by eljefino
... Watching it in Torque is fun; above 50 MPH when I coast the engine keeps running but at an 18:1 AFR instead of 14.7.

When the gas motor kicks on and off there's the tiniest surge in AFR but it settles and locks in very quickly.

This is of course the computer's opinion of what's going on. ...
Is that in a Prius? If so, are you sure that indicated 18:1 is real? Should be infinity-to-one when coasting fast enough that the engine is turning, because it shouldn't be injecting any fuel. (That's similar to fuel-cut mode while coasting in a modern non-hybrid.)
 
Originally Posted by CR94
Originally Posted by eljefino
... Watching it in Torque is fun; above 50 MPH when I coast the engine keeps running but at an 18:1 AFR instead of 14.7.

When the gas motor kicks on and off there's the tiniest surge in AFR but it settles and locks in very quickly.

This is of course the computer's opinion of what's going on. ...
Is that in a Prius? If so, are you sure that indicated 18:1 is real? Should be infinity-to-one when coasting fast enough that the engine is turning, because it shouldn't be injecting any fuel. (That's similar to fuel-cut mode while coasting in a modern non-hybrid.)


IIRC, torque won't go to infinity. When I would monitor it I would use the voltage graph though, it would just peg itself at the top
 
Originally Posted by The Critic
Originally Posted by bdcardinal
I might have done it when the head gasket was replaced on the off chance coolant contaminated the sensor.

I forgot to mention that. Good point.

Apparently, BMW recommends replacing Oxygen Sensors every 120K or 150K depending on the vehicle. I have heard this before and I thought it was a rumor, but apparently it is true. Take a look at the snapshot from the maintenance manual below - it was taken from the manual of a 2015 BMW 3-series. Very interesting...

I think this might be just a German owners manual thing. The 89 Cabriolet I had with Mechanical Fuel Injection (CIS) that was equipped with an Oxygen sensor to change the trims also had in its owners manual to change it at a similar interval.

On a side note that vehicle was really neat you could see the trims with an old Spark Gap Dwell meter hooked up to the Jetronic control box and tune the rich/lean swing. I managed to get the CO at idle down to 0.00 on my dad's 5 gas analyzer that is how good that Bosch fuel injection system was back then.
 
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Originally Posted by Skippy722
Originally Posted by CR94
Originally Posted by eljefino
... Watching it in Torque is fun; above 50 MPH when I coast the engine keeps running but at an 18:1 AFR instead of 14.7.

When the gas motor kicks on and off there's the tiniest surge in AFR but it settles and locks in very quickly.

This is of course the computer's opinion of what's going on. ...
Is that in a Prius? If so, are you sure that indicated 18:1 is real? Should be infinity-to-one when coasting fast enough that the engine is turning, because it shouldn't be injecting any fuel. (That's similar to fuel-cut mode while coasting in a modern non-hybrid.)


IIRC, torque won't go to infinity. When I would monitor it I would use the voltage graph though, it would just peg itself at the top


Narrow-band O2 sensors don't read down to zero either.
 
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