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So I got the dreaded P0420 Catalyst below threshold code 2 nights ago for the first time time since had the car in a year. Very well maintained for the 25k miles and counting since I put the motor in. When I check the data on the upstream 02's and it seems to be pulling a little fuel out not much. Also interestingly enough both banks are reading almost identical but it only threw that one code for bank 1. I threw a vacuum gauge on and the motor holds a super solid 22inhg and doesnt drop as you increase RPM. So no clogged cats...I smoke test it and i have zero leaks. Motor runs a smooth as can be. I have a bad rear motor mount and at idle you can't even feel the motor running lol. Now 2 things to mention.
1. The car is completely 100% bone stock not even CAI. I threw the car on the dyno just to see what it does and it made 216hp @ 202tq which for a 167k trans and a 125k motor thats great but there seems to be a very short area of dead zone around VTEC crossover. See the pic below. If anyone can chime in let me hear your thoughts on this.
2. (I know yall gunna kill me). The car as long as I had it ran on 87octane. That dyno run was with 87 octane. I recently went to 93octane say a week prior to the light coming on in the hopes to do another official stock baseline pull before throwing in cams. Could the change in octane have something to do with this p0420 code mysteriously popping up? I cleared the code the night it popped up and im waiting to see if it pops up again.
My verdict on this is it might just be bad reading upstream o2's. What do you guys think.
Last edited by fullboost833; Mar 1, 2020 at 12:57 PM.
So I got the dreaded P0420 Catalyst below threshold code 2 nights ago for the first time time since had the car in a year. Very well maintained for the 25k miles and counting since I put the motor in. When I check the data on the upstream 02's and it seems to be pulling a little fuel out not much. Also interestingly enough both banks are reading almost identical but it only threw that one code for bank 1. I threw a vacuum gauge on and the motor holds a super solid 22inhg and doesnt drop as you increase RPM. So no clogged cats...I smoke test it and i have zero leaks. Motor runs a smooth as can be. I have a bad rear motor mount and at idle you can't even feel the motor running lol. Now 2 things to mention..
Did you try putting a gauge on the exhaust? I don't have much faith on the vacuum gauge method.
Originally Posted by fullboost833
Could the change in octane have something to do with this p0420 code mysteriously popping up? I cleared the code the night it popped up and im waiting to see if it pops up again.
I doubt it was due to the change in octane. You'll need to complete the drive cycle that runs the catalyst monitor and see if it comes back.
Originally Posted by fullboost833
My verdict on this is it might just be bad reading upstream o2's. What do you guys think.
Did you look at B1S2 O2 sensor?
B1S2 is your cat monitoring sensor.
If you have access to a dyno, then you also probably have access to a capable scanner.
Did you try putting a gauge on the exhaust? I don't have much faith on the vacuum gauge method.
Did you look at B1S2 O2 sensor?
B1S2 is your cat monitoring sensor.
If you have access to a dyno, then you also probably have access to a capable scanner.
I do have a backpressure gauge but the exhaust was hot and i was being lazy. But you can see on both banks long term and short term are reading about the same. One thing for sure thought the upstream are a bit lazy i notice transitioning on and off throttle at different speeds. Thats why im thinking the sensors just need to be changed. I guess if you think its worth it to see if the rear cat is starting to clog I can give it a go.
Ive had my TL for about 6 years 199K miles and the p0420 is the code that my car gets at time. Weird part about it is that when the CEL comes on, after 2-3 days it always goes away and won’t come back for another couples weeks or months. Then it comes back for a day or two and goes away. It has done that for the last 4 years. I was always told if the check engine light don’t stick don’t worry about it.
I recently did replace both upstream o2 sensors and haven’t really seen the light yet (3 weeks now) kinda going off what this other gentleman said, the downsteam sensors are the one monitoring the flow. Sensor 1 is on top, sensor 2 is on the bottom of bank 1 & 2.
Use high tier gas for your car if you really care about it.
I’ve always use shell 93 ever since I owned the car, you gonna mess up the pistons in your engine.
I would suggest to see if it goes away by itself after couple days, if it doesn’t replace the sensors.
Originally Posted by fullboost833
So I got the dreaded P0420 Catalyst below threshold code 2 nights ago for the first time time since had the car in a year. Very well maintained for the 25k miles and counting since I put the motor in. When I check the data on the upstream 02's and it seems to be pulling a little fuel out not much. Also interestingly enough both banks are reading almost identical but it only threw that one code for bank 1. I threw a vacuum gauge on and the motor holds a super solid 22inhg and doesnt drop as you increase RPM. So no clogged cats...I smoke test it and i have zero leaks. Motor runs a smooth as can be. I have a bad rear motor mount and at idle you can't even feel the motor running lol. Now 2 things to mention.
1. The car is completely 100% bone stock not even CAI. I threw the car on the dyno just to see what it does and it made 216hp @ 202tq which for a 167k trans and a 125k motor thats great but there seems to be a very short area of dead zone around VTEC crossover. See the pic below. If anyone can chime in let me hear your thoughts on this.
2. (I know yall gunna kill me). The car as long as I had it ran on 87octane. That dyno run was with 87 octane. I recently went to 93octane say a week prior to the light coming on in the hopes to do another official stock baseline pull before throwing in cams. Could the change in octane have something to do with this p0420 code mysteriously popping up? I cleared the code the night it popped up and im waiting to see if it pops up again.
My verdict on this is it might just be bad reading upstream o2's. What do you guys think.
Ive had my TL for about 6 years 199K miles and the p0420 is the code that my car gets at time. Weird part about it is that when the CEL comes on, after 2-3 days it always goes away and won’t come back for another couples weeks or months. Then it comes back for a day or two and goes away. It has done that for the last 4 years. I was always told if the check engine light don’t stick don’t worry about it.
I recently did replace both upstream o2 sensors and haven’t really seen the light yet (3 weeks now) kinda going off what this other gentleman said, the downsteam sensors are the one monitoring the flow. Sensor 1 is on top, sensor 2 is on the bottom of bank 1 & 2.
Use high tier gas for your car if you really care about it.
I’ve always use shell 93 ever since I owned the car, you gonna mess up the pistons in your engine.
I would suggest to see if it goes away by itself after couple days, if it doesn’t replace the sensors.
Its a pending code I checked it yesterday. Im gunna throw some new upstream sensors in and see how it goes. Thanks.
As suspected it was just bad reading upstreams. So far ive gone 110miles+ and 10+ drive cycles and no pending codes where as before swapping sensors pending code would appear after ~10-15miles on about 2 drive cycles after clearing the code. Vacuum at idle Vacuum @2k rpm. If there was a clogged cat this number would be much less like in the 10inhg region. Front 02 Rear 02 Front cat Rear cat
[QUOTE=fullboost833;16546916]As suspected it was just bad reading upstreams. So far ive gone 110miles+ and 10+ drive cycles and no pending codes where as before swapping sensors pending code would appear after ~10-15miles on about 2 drive cycles after clearing the code.
well I’m glad it’s working out for you. I hope you used OEM O2 sensor (Densco) and not Bosch
