2001 V70XC - Intermittent ECM 4801

[abscate]

P0422 maps to the cat efficiency problem

P0420 doesnt show on the Volvo list?

[xHeart]

The front sensor display is NOT voltage on 2001 V70XC model. The front O2 sensor is a wideband sensor with a small bandwidth, usually less than 100mV. The ECM will compare the referenced control-signal with the output control-signal to display the value that VIDA calls lambda; it has NO unit associated with it. The lambda value is either O/L or C/L -- at times called Open Loop, Closed Loop, respectively.

Monitoring of a front wideband style O2 sensor makes little sense because it can't readily display either rich or lean conditions, hence the Lambda value is simplified to show C/L at a warm engine - cooling temp 83ºC. A direct read of O2 sensor using voltmeter will yield a low millivolt signal between oxygen reference, calibration, and/or output. It swaps polarity as the voltage switches above and below the reference value.

It is the rear O2 binary sensor that reports the level of oxygen stored in the cat at any time. The oxygen level in the cat changes during the rich and lean cycle, however a small quantity of oxygen always remains if catalyst is functioning. On acceleration the rear O2 sensor will show rich (>.75V) for catalyst function -- a diminishing oxygen level from burning any excess fuel in cat. Where, coasting would bring injector's fuel delivery to near stop, until engine gets down to idle speed -- the post cat sensor (rear O2) will register excess oxygen content (nearly 0v). This cycling of oxygen from .75v to 0v is normal. FWIW: The rear O2 sensor's value is .45v at idle on our 2001 V70XC.

Only the front wideband O2 sensor indicates faulty cat if the load is stable.!!!
When the load is stable the front sensor will oscillate within stoichiometric. In the case of front wideband O2 sensor, this means .97 to 1.03. When the exhaust gas is deviating from stoichiometric in a small range then the catalyst will function well, and the oxygen level post catalyst will be low leading to the .65-.75v, a value should be seen on rear binary sensor.

I would like to believe a Volvo catalytic converter by itself does not fail, therefore P0420 is never reported; unless someone says otherwise.

The above text from jimmy57 is a valuable source. It does take parsing -- a process of identifying clues within the long sentences, looking for recognizable and repeated patterns, and then matching it against the findings over VIDA/Dice.

I've edit jimmy57's text into paragraphs after parsing. Added punctuations, and shorter sentences to help captured the original intent and context. The edited text fits my findings.

Can someone confirm if the .45v value at the rear O2 sensor is normal - idling at either O/L or C/L?

[xHeart]

It is the rear O2 binary sensor that reports the level of oxygen stored in the cat at any time. The oxygen level in the cat changes during the rich and lean cycle, however a small quantity of oxygen always remains if catalyst is functioning. On acceleration the rear O2 sensor will show rich (>.75V) for catalyst function -- a diminishing oxygen level from burning any excess fuel in cat. Where, coasting would bring injector's fuel delivery to near stop, until engine gets down to idle speed -- the post cat sensor (rear O2) will register excess oxygen content (nearly 0v). This cycling of oxygen from .75v to 0v is normal.

FWIW: The rear O2 sensor's value is .45v at idle on our 2001 V70XC. Can someone confirm if the .45v value for the rear O2 sensor is normal at idling (no-load), either O/L or C/L, indicating presence of stored oxygen in catalytic converter?

[xHeart]

Either the front O2 sensor or injectors are a frequent cause of catalyst efficiency codes. The front sensor's signal is effectively a sum of exhaust gases from ALL cylinders. If it reports a wrong signal then the gases entering converter are catalyzed by catalyst.

An obstructed injector would make that cylinder lean due to the presence of excess oxygen. It would signal a lean condition at the front sensor, causing ECM to change injector's opening-time until the exhaust gases' oxygen content is within the range.

In another scenario, a rich cylinder can make up for the leanness of the obstructed injector. Whenever exhaust gases, other than oxygen portion, are in wrong concentrations the catalyst will do the task as it should.

Any exhaust leak at the head or turbo can make the front O2 sensor trigger efficiency code too.

^parsed... very helpful!

[xHeart]

Getting to know the oxygen sensor

The oxygen sensor measures the amount of oxygen in the exhaust that is used in the combustion process.

For pre-catalytic converter oxygen sensors used for fuel control:

Less oxygen than optimal in the exhaust results in a signal voltage over 450 mV. This reflects a RICH CONDITION. More oxygen in the exhaust than optimal results in a signal voltage under 450 mV. This reflects a LEAN CONDITION.

Good oxygen sensors have even waves in the 150 mV to 850 mV range while ascending or descending within a 100 mS or less when the system is in closed loop.

For post-catalytic converter oxygen sensors used for fuel control:

Post-cat oxygen sensors, when good, feature a steady voltage usually between 500 to 700 mV. If it zigzags, the catalytic converter is highly suspect.

On some vehicles the rear sensor does have some effect on fuel control. For our purposes, it’s just good to know that when testing the sensor, the voltage should go up when the fuel mixture is rich and should go down when it is lean. Sadly there is no way of generically knowing what is an optimal post-cat oxygen sensor voltage. It differs by the manufacturer.

Both front and rear oxygen sensors can be tested in the same way:

To make sure the sensor is reacting as it should to rich and lean conditions, simply cause a vacuum leak to make the system lean and use some propane to make the system run rich. You can do all of this by simply pulling out the brake booster hose. After you do this be sure to pump the brakes a couple times after you put everything back together. The sensor should react to rich and lean conditions instantly. If not, you might have a “lazy” sensor that needs to be replaced.

The bold text 450mV (.45v) brings some clarity to our 2001 V70XC's idle value of .45v at the rear sensor.
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