How to Know if It's Your Catalytic Converter or O2 Sensor
- rosario99 [OP]
- Sr. Member
- Apr 29, 2010
- 957 posts
- 82 upvotes
- London
Jul 8th, 2017 1:12 am
BAD Cat Converter OR O2 Sensor -- Which is it ??
Vehicle: 2002 Dodge Caravan SE (3.3, V6)
Fault Code: P0420 Catalyst System Efficiency Below Threshold
I attached an OBDII bluetooth sensor with Torque Pro and found the O2 Sensor Bank 1 Sensor 1 (upstream) is fluctuating wild (normal). As for O2 Sensor Bank 1 Sensor 2 (downstream), it is also fluctuating all over seeming to replicate Sensor 1 but slightly delayed. The mechanic told me to replace both O2 sensors to resolve the Fault Code but after seeing the O2 sensor readings doesnt this indicate i have a bad catalytic converter? Take note that I had a long time bad intake manifold leak and radiator leak leading to an overheated engine and some crazy hot / cold blowing from the in cabin blower. It is all resolved now but i wonder if this all contributed to killing my cat converter.
Can someone tell me what is the smoking gun indicator that i have a bad O2 sensor OR a bad cat converter. I dont want to replace both sensors if they arent the problem. And i certainly dont want to pay a fortune to replace the cat converter if it isnt needed. What would the sensor reading be for O2 Bank 1 Sensor 2 if this sensor were to go bad? Would it be a flat line / no fluctuation or would it be jumping around in readings matching Sensor 1?
The check engine light is on, of course. There is no rough idle.
Thx.
- #2
- prochargedv8
- Member
- Apr 29, 2017
- 426 posts
- 541 upvotes
Jul 8th, 2017 7:31 am
Take the sensors from bank 2 and swap them with bank 1, clear the code and drive around, if the code comes up as bank 1 again, you'll know it's the cat, if the code comes up as bank 2 it's the sensors. the sensors can be tested with a multimeter, checking resistance on each pin.
The fluctuations is usually because the pcm will run a cylinder bank rich and then back to stoich to test the cats efficiency and sensors.
- #3
- Pete_Coach
- Deal Expert
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- May 10, 2005
- 36250 posts
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- Ottawa
Jul 8th, 2017 8:06 am
Here is a good article on P0420. Misdiagnosis is common. http://www.easterncatalytic.com/educati ... o42o-code/
A problem with trouble codes is that people and even technicians take the simple way out. Sensors only report anomalies, if it does not "see" what it is programmed to see, then they turn on the light. What causes the sensor to see this is what needs to be checked and sometimes more tests are needed.
Thing is, most people do not understand the system well enough to do the investigation and many techs are too lazy and eager to sell parts to delve into the real problem.
So, is it a cat or a sensor or, is it some other problem (fuel ?) that causes the sensor to "feel" an anomaly?
- #4
- tmkf_patryk
- Deal Addict
- Dec 27, 2007
- 3787 posts
- 1290 upvotes
- Edmonton
Jul 8th, 2017 12:23 pm
Easiest way is, if your readings on sensor 1 and sensor 2 are the same (fluctuating) then it's your cat converter. If 2 is fairly stable, then it's doing a good job and there is no problem. If the flat line is 0 or some absolute (I believe it's a 5v max) then it's sensor
warming up the earth 1 gas fill-up at a time...
You only live once, get a v8
- #5
- KosMos
- Deal Addict
- Jan 13, 2009
- 1562 posts
- 190 upvotes
- Oakville
Jul 8th, 2017 2:04 pm
rosario99 wrote: ↑ I attached an OBDII bluetooth sensor with Torque Pro and found the O2 Sensor Bank 1 Sensor 1 (upstream) is fluctuating wild (normal). As for O2 Sensor Bank 1 Sensor 2 (downstream), it is also fluctuating all over seeming to replicate Sensor 1 but slightly delayed.
Was in the same boat about a month ago.
Googled a LOT.
Most likely it is cat. I'm suggesting do not waste money on sensors.
The question is - do you want to fix the problem or just pass emissions? 
If former - replacing cat most likely would work.
If latter - just google "cheap fix for P0420" and you and you will find a trick (code word - "non-foulers").
Good luck.
- #6
- hoob
- Deal Fanatic
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- Jan 6, 2002
- 6088 posts
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- Toronto
Jul 8th, 2017 2:13 pm
KosMos wrote: ↑ If latter - just google "cheap fix for P0420" and you and you will find a trick (code word - "non-foulers").
Yup. Non-fouling tube gets rid of the problem a lot (worked great in my RAV4.)
Keep in mind you would probably need to install it yourself. Professional mechanics are forbidden to tamper with emissions controls with some rather bad penalties, but there's really no recourse if vehicle owner does it.
Also note that if you ever trade the vehicle in or it is inspected for sale, it might be flagged as unacceptable and need to be fixed at the time. But if you are the last owner of the vehicle, you're good to go (aside from the very small amount of shame you will have because you are polluting more than you "should" 
)
As someone long prepared for the occasion, in full command of every plan you wrecked---
Do not choose a coward's explanation, that hides behind the cause and the effect...
- #7
- billford
- Deal Addict
- Feb 6, 2011
- 1814 posts
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Jul 8th, 2017 5:56 pm
rosario99 wrote: ↑
I attached an OBDII bluetooth sensor with Torque Pro and found the O2 Sensor Bank 1 Sensor 1 (upstream) is fluctuating wild (normal). As for O2 Sensor Bank 1 Sensor 2 (downstream), it is also fluctuating all over seeming to replicate Sensor 1 but slightly delayed.
If precat sensor is operating correctly, crosscounts, amplitude, fuel trims, response time and post cat sensor pattern is the same as precat,the problem is inbetween the 2 sensors. Cat converter.
Make sure there are no engine problems first, otherwise the new converter will be damaged.
- #8
- Pete_Coach
- Deal Expert
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- May 10, 2005
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Jul 9th, 2017 6:57 am
billford wrote: ↑ If precat sensor is operating correctly, crosscounts, amplitude, fuel trims, response time and post cat sensor pattern is the same as precat,the problem is inbetween the 2 sensors. Cat converter.
Make sure there are no engine problems first, otherwise the new converter will be damaged.
Funny, real funny.
Perhaps you can explain to the OP how you would diagnose and test for those faults to be sure LOL.
- #9
- billford
- Deal Addict
- Feb 6, 2011
- 1814 posts
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Jul 9th, 2017 3:01 pm
Pete_Coach wrote: ↑ Funny, real funny.
Perhaps you can explain to the OP how you would diagnose and test for those faults to be sure LOL.
Op can graph with torque, which they already have. There are many instructional videos already available on how to interpret the results.
Although torque is slower than a scope, it still can be used as diy.
- #10
- Pete_Coach
- Deal Expert
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- May 10, 2005
- 36250 posts
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Jul 9th, 2017 3:16 pm
billford wrote: ↑ Op can graph with torque, which they already have. There are many instructional videos already available on how to interpret the results.
Although torque is slower than a scope, it still can be used as diy.
Har Har Har, you certainly are a comedian
- #11
- billford
- Deal Addict
- Feb 6, 2011
- 1814 posts
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Jul 9th, 2017 3:18 pm
Pete_Coach wrote: ↑ Har Har Har, you certainly are a comedian
Don't understand your comment, please explain.
- #12
- Pete_Coach
- Deal Expert
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- May 10, 2005
- 36250 posts
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Jul 9th, 2017 3:20 pm
billford wrote: ↑ Don't understand your comment, please explain.
Not until you explain your post LOL
- #13
- billford
- Deal Addict
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- 1814 posts
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Jul 9th, 2017 3:24 pm
Pete_Coach wrote: ↑ Not until you explain your post LOL
Have you ever used torque?
- #14
- rosario99 [OP]
- Sr. Member
- Apr 29, 2010
- 957 posts
- 82 upvotes
- London
Jul 13th, 2017 11:10 am
KosMos wrote: ↑ Was in the same boat about a month ago.
Googled a LOT.
Most likely it is cat. I'm suggesting do not waste money on sensors.
The question is - do you want to fix the problem or just pass emissions?
If former - replacing cat most likely would work.
If latter - just google "cheap fix for P0420" and you and you will find a trick (code word - "non-foulers").
Good luck.
Fascinating trick to get rid of the Check Light. Still confused how a "coupler" turns it off. But i intend on taking the vehicle in for an E-Test. Even with the check engine light off (== sensor tricked) wont the car still fail the test since the exhaust output will still fail standards??
- #15
- jaswest2754
- Member
- Aug 12, 2016
- 379 posts
- 266 upvotes
Jul 13th, 2017 1:21 pm
rosario99 wrote: ↑ Fascinating trick to get rid of the Check Light. Still confused how a "coupler" turns it off. But i intend on taking the vehicle in for an E-Test. Even with the check engine light off (== sensor tricked) wont the car still fail the test since the exhaust output will still fail standards??
No because it just relies on the information your car computer generates. You can check it yourself to see if it will pass thru the torque app. It will say if the emission system is ready or not. There is no more tail pipe test for cars with an OBD2 connector.
- #16
- rosario99 [OP]
- Sr. Member
- Apr 29, 2010
- 957 posts
- 82 upvotes
- London
Jul 13th, 2017 7:35 pm
jaswest2754 wrote: ↑ No because it just relies on the information your car computer generates. You can check it yourself to see if it will pass thru the torque app. It will say if the emission system is ready or not. There is no more tail pipe test for cars with an OBD2 connector.
Ahh i didnt know that. Thanks. So its years 1998 to present which only rely upon the OBDII results whereas 1997 and earlier is a tailpipe test. Good to know. I will now for sure check out this non-fouler.
- #17
- engineered
- Deal Expert
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- Feb 11, 2007
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- GTA
Jul 13th, 2017 9:54 pm
rosario99 wrote: ↑ Ahh i didnt know that. Thanks. So its years 1998 to present which only rely upon the OBDII results whereas 1997 and earlier is a tailpipe test. Good to know. I will now for sure check out this non-fouler.
If the shop is thorough, they may see the non-fouler and fail you. They're supposed to inspect the car for non-standard/missing emissions equipment.
If the women don't find you handsome, they should at least find you handy.
- #18
- jaswest2754
- Member
- Aug 12, 2016
- 379 posts
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Jul 13th, 2017 10:23 pm
engineered wrote: ↑ If the shop is thorough, they may see the non-fouler and fail you. They're supposed to inspect the car for non-standard/missing emissions equipment.
Very good wording. Well they are supposed to I doubt they did when I took my car in. They didn't put it on the hoist. I watched the guy sit in my car connect the thing wait a couple min, get out print the piece of paper and off I went after I paid. He didn't even open the hood. This is YMMV though as my tech or whatever he may of been, may have just been lazy. Your emission bypass equipment could possibly be seen if they perform a visual inspection.
- #19
- engineered
- Deal Expert
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- Feb 11, 2007
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Jul 13th, 2017 10:55 pm
jaswest2754 wrote: ↑ Very good wording. Well they are supposed to I doubt they did when I took my car in. They didn't put it on the hoist. I watched the guy sit in my car connect the thing wait a couple min, get out print the piece of paper and off I went after I paid. He didn't even open the hood. This is YMMV though as my tech or whatever he may of been, may have just been lazy. Your emission bypass equipment could possibly be seen if they perform a visual inspection.
Agreed. They might take a peek under the hood for anything unusual, but probably not much more unless your car is riced out. That said I can see my O2 sensors from above the engine bay.
If the women don't find you handsome, they should at least find you handy.
- #20
- rosario99 [OP]
- Sr. Member
- Apr 29, 2010
- 957 posts
- 82 upvotes
- London
Jul 14th, 2017 3:53 pm
UPDATE
I tried the non-fouler a suggested and what the heck, so far it is working!! Impressive hack. Im just waiting on the O2 Sensor Heater (still incomplete) but all the others are complete with no check light on yet. Will keep driving and power cycling. Good stuff, guys! Will let u know if the car passes
How to Know if It's Your Catalytic Converter or O2 Sensor
Source: https://forums.redflagdeals.com/bad-cat-converter-o2-sensor-2111204/