P1520 Clarification

[projectSHO89]

Look guys, your missing the point. A catalyst aids in chemical reactions. Any storage taking place there is a very tiny bit of what is happening overall. If you think that's the main way the cat functions, releasing stored oxygen most of the time while driving, you won't understand fully how the codes are set.......

No. I do have a very good understanding of what is supposed to set the code(s). The primary catalyst in a TWC is a compound either platinum, rhodium, or a combination. The cerium is added to the washcoat that is applied to the substrate is specifically for its Oxygen Storage Capacity (OSC). The primary catalyst breaks down nitrous oxides and volotile unburned hydrocarbons (VOCs) into less obnoxious compounds including CO, CO2, H2O, and O2. The O2 tends to be captured by the cerium, especially during lean engine operations. It is then released during rich operations or steady-state operation. This results in the analog to an electronic RC integrator.

Primary catalyst efficiency is an inferred value since it cannot be directly measured. The O2 concentrations in the exhaust, before and after the catalyst, are used to infer the condition of the catalyst. A dead catalyst will pass through oxygen and the relative levels will remain unchanged when measured by the downstream vs upstream o2 sensors. A "dying" or deteriorating catalyst will show some change in the O2 component of the exhaust stream relative to the inlet. Hence, the switch ratio is the indicator of how well the primary catalyst is functioning.

You are absolutely correct in that a marginal component in the system can result in a borderline condition. Unless a technician takes the time to look at more than just the DTC, there's a pretty fair percentage of the time that the snapshot diagnosis will be incorrect.

But evidently you know something that doesn't match the experience of many of us here.

I can only relate my own experience and the results of talking with two Ford master mechanics about this issue. The first was a friend of a family member who was the one who actually performed the precat swap on my 98 while it was in warranty. He connected the NGS system up to the car, looked at the O2 sensor waveforms (which I had already done), and went and ordered both precats for me. This was about 74,000 miles ago. The second mechanic is a classmate of mine from high school. Part of our recent discussion concerned this particular issue. He also confirmed that prior problems with misfires and over-rich conditions often caused these codes to come up after a period of time. It was his specific and multiple experiences that the precat often had to be replaced even after the primary problem was corrected.


As always, your mileage may vary.

Steve

 

[Big Jim]

No. I do have a very good understanding of what is supposed to set the code(s). The primary catalyst in a TWC is a compound either platinum, rhodium, or a combination. The cerium is added to the washcoat that is applied to the substrate is specifically for its Oxygen Storage Capacity (OSC). The primary catalyst breaks down nitrous oxides and volotile unburned hydrocarbons (VOCs) into less obnoxious compounds including CO, CO2, H2O, and O2. The O2 tends to be captured by the cerium, especially during lean engine operations. It is then released during rich operations or steady-state operation. This results in the analog to an electronic RC integrator.

This would probably better be stated "Some of the 02 tends . . ." There is no way that a significant volume of 02 can be captured on the substrate compared with the amount of 02 that goes through an engine. The rest of that is correct.

Primary catalyst efficiency is an inferred value since it cannot be directly measured. The O2 concentrations in the exhaust, before and after the catalyst, are used to infer the condition of the catalyst. A dead catalyst will pass through oxygen and the relative levels will remain unchanged when measured by the downstream vs upstream o2 sensors. A "dying" or deteriorating catalyst will show some change in the O2 component of the exhaust stream relative to the inlet. Hence, the switch ratio is the indicator of how well the primary catalyst is functioning.

"Hence, the switch ratio is the indicator of how well the primary catalyst is functioning" This is the most significant part according to both my Bosch and Ford training. The difference in the oxygen level isn't that much, but the dampened rapid fluctuation of the oxygen sensor readings is dramatic. A properly working upstream sensor switches very rapidly, and when the downstream sensor too closely mirrors the upstream sensor, the cat code is set.

You are absolutely correct in that a marginal component in the system can result in a borderline condition. Unless a technician takes the time to look at more than just the DTC, there's a pretty fair percentage of the time that the snapshot diagnosis will be incorrect.




I can only relate my own experience and the results of talking with two Ford master mechanics about this issue. The first was a friend of a family member who was the one who actually performed the precat swap on my 98 while it was in warranty. He connected the NGS system up to the car, looked at the O2 sensor waveforms (which I had already done), and went and ordered both precats for me. This was about 74,000 miles ago. The second mechanic is a classmate of mine from high school. Part of our recent discussion concerned this particular issue. He also confirmed that prior problems with misfires and over-rich conditions often caused these codes to come up after a period of time. It was his specific and multiple experiences that the precat often had to be replaced even after the primary problem was corrected.

As far as I can remember (it has been a few years) you could not read the waveform on NGS. The NGS system had a significant upgrade about two years ago, so that may have changed. IIRC you can read values, but not as a waveform.


As always, your mileage may vary.

Very true. My esperience has been that Contours set a lot of "false codes" for cat effeciency. Enough so that is is worth catching up on maintenance and resetting the light before spending money on a cat.

Steve

And since my edits were within the message, I must add 10 letters before posting.

 

[JaketheSnake]

you guys are the wind beneath my wings

 

[projectSHO89]

..and we now return you to your irregularly scheduled Contoru/Mystique failures....

;^)

Steve

 

[Tony2005]

LOL. It has been very diplomatic so far and I appreciate it. But this thread is done. Thank you to all who contributed.