Reply With QuoteThis can be a hard one sometimes. On my 99 we ended up having to replace the cat to make it go away.
P0420 Code: Help
My Scanner is saying the CEL that im throwing is a PO420, it also says Catalyst Inefficiency below threshold ( Bank 1)
Is that telling me my front 02 sensor is bad ? the one coming off the maniold?
Or the rear 02 sensor that it on the converter it self, (I would assume that one would be bank 2?)
Any help would be appreciated im trying to pass inspection and it would help if i could narrow down the correct one, id hate to change both if i don't have to.
This can be a hard one sometimes. On my 99 we ended up having to replace the cat to make it go away.
'01 Miata. Silver/Red Stripes. HDHCDD
'13 Mazda2. Spirited Green.
You have an o2 sensor before the cat, and one after the cat. If the cat is doing it's job, the ECU will see different reading between the 2 o2 sensors. Right now the ECU is saying that there is no difference, or not a big enough difference. The most likely cause is a bad cat. It is possible that the second sensor is bad, but IIRC, that would cause additional codes.
As for Bank 1 / Bank 2... That really only applies to engines with multiple exhaust manifolds (like a V. There is no bank 2 on a Miata.
Some advice- A bad 02 sensor can kill a cat. If your o2 sensors are old and you just replace your cat, there is a chance that when the o2 sensor goes out a year from now, it kills your expensive new cat. O2 sensors are pretty cheap, so I recommend replacing the cat and sensor at the same time.
Side note: I just went through all of this with my V8 Tahoe a few months ago. It has 2 cats and 4 o2 sensors, and the bank information told me the problem was only on one side. Being that all of the parts had 140k miles on it, I just went ahead and replaced both cats and all 4 sensors so I don't have to mess with it again for a long time. Code has not come back.
Last edited by Titus; 11-26-2011 at 11:32 AM.
On my '99 I replaced the cat, but the entire mid-pipe was rusted out too.
To debug this I found I had to look at the voltages from both O2 sensors. At idle the front sensor should move between IIRC .4 volts and 1.2 volts in a rhythmic pattern as the ECU fluctuates between lean and rich. The voltage on the 2nd O2 sensor should remain constant IIRC at .8 volts. I found the debug proceedure on miata.net. You'll have to search for it.
Many OBD-II adapters will allow monitoring the sensor voltage. I got a BlueTooth OBD-II adapter (PDX) and a free Android app "alOBD Scanner". With the "Torque" app, while it had lots of cool dials, I could not figure out how to display the voltage.
'99 Emerald Green - 2015 NASA Texas TTE Season Champion (showing up is really 100% of it)
It is very easy to get the voltage with a real scanner. You can borroow mine if you don't mine driving all the way across the county line. I am on the east side of Dallas county line. HF has this scanner on sale. I find the scanner very useful even for a TCB like me.
Do you have any leak in the rusted mid-pipe? It could be a leak there and make your 2nd O2 sensory reading too low and possibly not even cycle the way the ECU is programed to see. Readings from the 2nd O2 sensor should tell you this.
Gabriel
I forgot to mention, the last owner had a weld in replacement cat installed, i bought the car with the CEL On, he told me just clear it and it will go away, because i think that's what the guy at the exhaust shop told him.
So when i got home, I cleared it and for the past year it comes on every few days.
Luckily last year i cleared it and drove about 60-80 miles with out it coming on so i went to the Inspection station and it passed, this year im not so lucky i can barley go 50 miles without the code coming on.
i have just left it on the past year.
So a bad 02 sensor would throw a different code completely wouldn't it ?
There are a ton of generic O2 sensor specific codes out there. I am sure that not all apply to a Miata. If you are repeatedly throwing the cat efficiency code, while never throwing an o2 sensor code, I would be surprised if you had a bad o2 sensor and a good cat. I have heard that it can happen, but not very likely.
- P0030 HO2S Heater Control Circuit (Bank 1 Sensor 1)
- P0031 HO2S Heater Control Circuit Low (Bank 1 Sensor 1)
- P0032 HO2S Heater Control Circuit High (Bank 1 Sensor 1)
- P0036 HO2S Heater Control Circuit (Bank 1 Sensor 2)
- P0037 HO2S Heater Control Circuit Low (Bank 1 Sensor 2)
- P0038 HO2S Heater Control Circuit High (Bank 1 Sensor 2)
- P0053 HO2S Heater Resistance (Bank 1, Sensor 1)
- P0054 HO2S Heater Resistance (Bank 1, Sensor 2)
- P0130 02 Sensor Circuit Malfunction (Bank I Sensor 1)
- P0131 02 Sensor Circuit Low Voltage (Bank I Sensor I)
- P0132 02 Sensor Circuit High Voltage (Bank I Sensor 1)
- P0133 02 Sensor Circuit Slow Response (Bank 1 Sensor 1)
- P0134 02 Sensor Circuit No Activity Detected (Bank I Sensor 1)
- P0135 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
- P0136 02 Sensor Circuit Malfunction (Bank I Sensor 2)
- P0137 02 Sensor Circuit Low Voltage (Bank I Sensor 2)
- P0138 02 Sensor Circuit High Voltage (Bank I Sensor 2)
- P0139 02 Sensor Circuit Slow Response (Bank 1 Sensor 2)
- P0140 02 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
- P0141 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
Yeah its always a po4020,
So i guess im looking at getting a new cat ?
Any recommendations he already had one welded on, any other good weld on cats the previous owner may have bought a cheap one or something and that may have been why it failed in less then a month.
Im really not sure.
You still might want to take the time to test the voltages of the o2 sensors as others have suggested just to be sure...
As for a weld on cats, I don't have a recommendation. It looks like RockAuto will sell you a complete midpipe with cat in it (so no welding needed) for $250, and you should be able to find a 5% off code with a quick google search.
Cool, thanks for the help guys.Originally Posted by Titus
I replaced my original mid-pipe and muffler with a Roadster Sport mid-pipe and Racing Beat single tip muffler. They weren't that much more $ than the stock size and getting them from a local vendor (TDR) saved shipping. The quality of materials look better than stock. If they last 12 years, I'll have gotten my moneys worth.
It was relatively easy to change out, 4 bolts and 4 hangers. I sprayed the bolts down for a couple of days with Deep Creep and used Simple Green on the hangers.
'99 Emerald Green - 2015 NASA Texas TTE Season Champion (showing up is really 100% of it)
when my 97 was throwing the P0420 code i replaced the cat with a new bolt in one from RockAuto, my rear O2 and anything else i could find that had to do with the code. cleared it multiple times and it still came back... ended up beating the shit out of my car for about a week and it never came back on for about a year... maybe try clearing it and beating the shit out of it for a bit lol.
I beat the shit out of it for about 60 miles took it into the inspection station and the guy said my computer showed "ready" so he did the insepction on it, but during the count down he said it was taking too long and the car would fail
I don't understand what he was trying to say, he said try driving it a little more before the retest.
does this mean by cat is truly bad, even after having it replaced last year ?
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