Post Cat O2 Sensor
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From: SoCal & Lake Havasu, AZ
Post Cat O2 Sensor
I know that the post cat oxygen sensor does not make a true measure of the air fuel ratio. What does its signal mean? I hooked up my A/F gauge to it, and I get a pretty steady Stoich reading during normal cruise. Sometimes it read very lean at idle. Since I have a switch to toggle between the Pre cat sensor, I know that I get "normal" readings from it, but they don't seem to correlate as I toggle between the 2.
I don't want to be flamed about how worthless these gauges are, I was just curious about the signal that the 2nd cat sends. What is it measuring?
I plan to send the gauge to GadgetSeller to get it calibrated for my procharger, so this is really just a "HUH?"
Thanks to the experts that I know will respond to this.
I don't want to be flamed about how worthless these gauges are, I was just curious about the signal that the 2nd cat sends. What is it measuring?
I plan to send the gauge to GadgetSeller to get it calibrated for my procharger, so this is really just a "HUH?"
Thanks to the experts that I know will respond to this.
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From: SoCal & Lake Havasu, AZ
Originally posted by Quintin
The only thing the post cat O2s do is measure catalyst
The only thing the post cat O2s do is measure catalyst
Last edited by desparado; May 11, 2005 at 12:32 PM.
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From: minnesota
the back o2's dont do anything concerning the engine. THey will not adjust air fuel mixture or anything. They just montiter if your cats are bad or not. The post o2's will only come on with a defective cat, clogged cat, or dented. If your cats are working fine then they shouldnt ever throw a code (unles the o2 is bad) jsut curious to why you asked? did u throw a cel light? how mayn miles on the truck?
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From: SoCal & Lake Havasu, AZ
Thanks for your reply. I am just terminally curious. I connected my A/F gauge to one through a toggle so I could switch from front to back just to see what the reading would be. Since the back cat sends a signal that the gauge reads, I was curious as to what voltage it would sent to the PCM if it were going bad. I have no error codes. Truck runs like a dream with 80,000 miles. No problems, just my insatiable desire to learn what stuff does/means.
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From: minnesota
ha sounds like me except i screw with snowmobiles
my dad always said if it aint broke dont fix it...but what if it breaks and u NEED to fix it fast i like to kno how everything works on stuff that i will be fixing myself anyways
my dad always said if it aint broke dont fix it...but what if it breaks and u NEED to fix it fast i like to kno how everything works on stuff that i will be fixing myself anyways
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From: St. Louis (Out in the woods)
Perhaps this quote from the PDEC will explain it for you.
The Federal Test Procedure Catalyst Monitor monitors the catalyst system for deterioration and illuminates the MIL when tailpipe emissions exceed the appropriate HC emission thresholds. It is called the FTP catalyst monitor because it must complete during a standard emission test (the Federal Test Procedure). This monitor relies on the front and rear heated oxygen sensors (HO2S) to infer catalyst efficiency based upon oxygen storage capacity. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the rear HO2S quite slow compared with the frequency of the front HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines, and the rear HO2S begins to switch more rapidly, approaching the frequency of the front sensor. In general, as catalyst efficiency decreases, the switch ratio increases from a switch ratio of 0 for a low mileage catalyst to a switch ratio of 0.8 or 0.9 for a low-efficiency catalyst.
Many Low Emission California vehicles will monitor substantially less than the entire catalyst volume in order to meet the stringent catalyst monitoring malfunction thresholds. In many cases, only the front, light-off catalyst is monitored.
1. Front and rear HO2S switches are counted under specified closed loop fuel conditions. After the required number of front switches are obtained, a rear-to-front HO2S switch ratio is calculated. The switch ratio is compared against a threshold value. If the switch ratio is greater than the emission threshold, the catalyst has failed. Inputs from the ECT or CHT (warmed up engine), IAT (not at extreme ambient temperatures), MAF (greater than minimum engine load), VSS (within vehicle speed window) and TP(at part throttle) are required to enable the Catalyst Efficiency Monitor.
2. The DTCs associated with this test are DTC P0420 (Bank 1) and P0430 (Bank 2). Because an Exponentially Weighted Moving Average is used for malfunction determination, up to six driving cycles may be required to illuminate the MIL.
If you need further explanation, just ask.
What it boils down to, is that the post-cat sensor will, when operating correctly, will be an average of the readings of the front O2 sensor (and other factors).
Steve
The Federal Test Procedure Catalyst Monitor monitors the catalyst system for deterioration and illuminates the MIL when tailpipe emissions exceed the appropriate HC emission thresholds. It is called the FTP catalyst monitor because it must complete during a standard emission test (the Federal Test Procedure). This monitor relies on the front and rear heated oxygen sensors (HO2S) to infer catalyst efficiency based upon oxygen storage capacity. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the rear HO2S quite slow compared with the frequency of the front HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines, and the rear HO2S begins to switch more rapidly, approaching the frequency of the front sensor. In general, as catalyst efficiency decreases, the switch ratio increases from a switch ratio of 0 for a low mileage catalyst to a switch ratio of 0.8 or 0.9 for a low-efficiency catalyst.
Many Low Emission California vehicles will monitor substantially less than the entire catalyst volume in order to meet the stringent catalyst monitoring malfunction thresholds. In many cases, only the front, light-off catalyst is monitored.
1. Front and rear HO2S switches are counted under specified closed loop fuel conditions. After the required number of front switches are obtained, a rear-to-front HO2S switch ratio is calculated. The switch ratio is compared against a threshold value. If the switch ratio is greater than the emission threshold, the catalyst has failed. Inputs from the ECT or CHT (warmed up engine), IAT (not at extreme ambient temperatures), MAF (greater than minimum engine load), VSS (within vehicle speed window) and TP(at part throttle) are required to enable the Catalyst Efficiency Monitor.
2. The DTCs associated with this test are DTC P0420 (Bank 1) and P0430 (Bank 2). Because an Exponentially Weighted Moving Average is used for malfunction determination, up to six driving cycles may be required to illuminate the MIL.
What it boils down to, is that the post-cat sensor will, when operating correctly, will be an average of the readings of the front O2 sensor (and other factors).
Steve
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From: SoCal & Lake Havasu, AZ
Thanks projectSHO89
Now that's what I'm talkin about. Great answer that confirms my suspicions and the steady stoich reading I get in cruise from that sensor.
My life is near complete.
Now that's what I'm talkin about. Great answer that confirms my suspicions and the steady stoich reading I get in cruise from that sensor.
My life is near complete.