Can poor quality gas cause a P0420 (Cat Failure)?

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Vehicle: 1997 Grand Caravan, 3.3l, 160k miles.

This morning I did something I hardly ever do: I let the tank get down to almost empty and then had to fuel up at a dumpy gas station. Anyway, later today I got a "Service Engine Soon" light. So I get the code(s), P0420 & MIL12, and find out that the P0420 is the generic "catalyst efficiency below required level." Naturally, panic set in: New cat, new upstream/downstream sensors, etc. But I drove the vehicle for the rest of the day and there are no obvious problems (no rough idle, no hesitation, no loss of power)...it runs same as ever.

That said, could filling an almost empty tank with bad gas cause the PCM to throw this code? If so, and it'll take me 2 weeks to burn through this tank of gas, which additive should I use to mitigate the problem? Techron, MMO, Lucas, etc?

Thanks for any help!
 
From my Caravan Service Manual, Catalyst monitor section:

Quote:
To monitor the system, the number of lean-to-rich switches of upstream and downstream O2S's is counted. The ratio of downstream switches to upstream switches is used to determine whether the catalyst is operating properly. An effective catalyst will have have fewer downstream switches than it has upstream switches i.e., a ratio closer to zero. For a totally ineffective catalyst, this ratio will be one-to-one, indicating that no oxidation occurs in the device.

The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over the legal limit, the MIL (check engine lamp) will be illuminated.


Based on that explanation, I don't think bad gas is factor, but I could be wrong.
 
Don't bother replacing the downstream O2 sensor to fix this. It won't help. Replacing the upstream O2 sensor might help the replacement cat last longer.
 
I've seen new vehicles throw an errant catalyst inefficiency code, that never came back once cleared, but 13yrs/old & 160Kmi, your cats are probably done. You're still going to want to clear it & see if it comes back though. How many cats does the 3.3L have? The code should pinpoint which bank (1 or 2). If the exhaust system as a whole is in excellent shape yet, I'd try replacing the upstream sensor on the bank that's flagged and hope + pray that it yields a ratio that the OBDII system likes.

Joel
 
Originally Posted By: brianl703
Don't bother replacing the downstream O2 sensor to fix this. It won't help. Replacing the upstream O2 sensor might help the replacement cat last longer.


Yep. I'd try the upstream O2 sensor and then clear the code. If the code returns it's time for a new cat. I definitely wouldn't replace the cat without testing/replacing the pre-cat O2 sensor.
 
A PO420 does not automatically mean a mean a bad cat.
A lazy downstream O2 sensor (not bad enough to throw a O2 code),minor exhaust and vacuum leaks are good candidates for this code also.

Before replacing any parts the cat can be checked by checking the temperature difference between the inlet and outlet of the cat with an infra-red thermometer.Look for minimum 30F difference @2000rpm Higher readings i.e 200f indicate rich mixture.

Testing the O2 sensors can be done fairly easily with an appropriate scan tool.

Try a bottle of Gumout all in one first.
 
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Thanks to everyone for the suggesions/tips.

I reset the light this morning and took the van out on a good 20 minute highway drive; no CEL so far. However, I did start looking online for an upstream sensor, as that's the cheapest place to start, and found a Bosch 15705 w/OE connector for $34 on Amazon (bought it); Autozone & O'Reilly's wanted $64+ for the same part. If that doesn't do it, I'll try the downstream sensor next (Amazon/Bosch 13149 is $45)...and then the cat (they're goint to want to replace the downstream sensor anyway when they install the new cat). But hopefully it won't go that far.

M_C
 
Originally Posted By: Trav

A lazy downstream O2 sensor (not bad enough to throw a O2 code)


Explain that one. The ECU caculates a switch ratio by dividing the number of downstream switches by the number of upstream switches. If it exceeds a certain value (.8 on my Crown Vic), a P0420 or P0430 code is set.

A lazy downstream (post cat) sensor would, in fact, reduce the switch ratio, making it less likely that a P0420 or P0430 code is set.

Quote:
..and then the cat (they're goint to want to replace the downstream sensor anyway when they install the new cat)


I wouldn't spend the money to replace the downstream sensor. There's no point to it. It's obviously working well enough if it can switch fast enough to generate a P0420 or P0430 code.
 
Originally Posted By: brianl703

I wouldn't spend the money to replace the downstream sensor. There's no point to it. It's obviously working well enough if it can switch fast enough to generate a P0420 or P0430 code.

Fair enough.

BTW: How are MagnaFlow cats regarded? Amazon is listing one for my van at $120. That seems awfully cheap, but then I don't need an OEM replacement...the van only gets about 5k miles of use per year, I'm good on emissions testing until Fall of 2012 and I don't plan on keeping it for too much longer after that.
 
Sometimes we would get a p420 on the explorers and expiditions .Ford had a tsb that said to reprogram the pcm before condemning the cats you might want to check with a local dealer to see if there is a tsb for a reflash.
 
Originally Posted By: Familyguy

Yep. I'd try the upstream O2 sensor and then clear the code. If the code returns it's time for a new cat. I definitely wouldn't replace the cat without testing/replacing the pre-cat O2 sensor.


+1
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This is most likely a bad cat or downstream sensor problem.
Because it's the second sensor that tells the computer the cat is bad.
But if you get a new cat, I'd get both sensors, for a bunch or reasons.
It's probably a coincidence about the recent fill up. You have a lot of miles on those parts, and they are normally due for replacement to work their best now.
 
Originally Posted By: brianl703
Trav said:
A lazy downstream O2 sensor (not bad enough to throw a O2 code)


Explain that one. The ECU caculates a switch ratio by dividing the number of downstream switches by the number of upstream switches. If it exceeds a certain value (.8 on my Crown Vic), a P0420 or P0430 code is set.

A lazy downstream (post cat) sensor would, in fact, reduce the switch ratio, making it less likely that a P0420 or P0430 code is set.

Quote:
..and then the cat (they're goint to want to replace the downstream sensor anyway when they install the new cat)

I wouldn't spend the money to replace the downstream sensor. There's no point to it. It's obviously working well enough if it can switch fast enough to generate a P0420 or P0430 code.



Read for yourself.There are many references to the PO420 code and Downstream O2 sensor issues to be found in any emission book.
http://www.aa1car.com/library/o2sensor.htm
http://www.aa1car.com/library/p0420_dtc.htm
 
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Originally Posted By: Trav


Read for yourself.There are many references to the PO420 code and Downstream O2 sensor issues to be found in any emission book.
http://www.aa1car.com/library/o2sensor.htm
http://www.aa1car.com/library/p0420_dtc.htm


I've read lots about this, including the Ford TSB where it's stated that replacing the downstream sensor WILL NOT correct a P0420/P0430 issue.

Nothing you linked to suggests that a faulty downstream sensor will cause a P0420/P0430, only that a faulty O2 sensor will affect the accuracy of the catalyst monitor test. Which is true, but (in the case of a faulty downstream sensor) in favor of passing a degraded catalyst, not the other way around...and that's assuming the faulty downstream sensor doesn't get detected as such and a code set for that.

In order for a faulty downstream O2 sensor to cause a P0420 or P0430 code, it would have to fail in a way that causes it to switch faster. That's not how O2 sensors fail.
 
Quote:
If the downstream O2 sensor is bad (heater circuit not working, loose or corroded wiring connector, contaminated sensor element, etc.), the OBD II system should detect the fault and set an oxygen sensor code. The same goes for a bad upstream O2 sensor. In either case, the presence of an O2 sensor code should prevent the catalyst monitor from running and setting a false P0420 code. Of course, this is ideally speaking and nothing is ever ideal. Sometimes a faulty O2 sensor is not bad enough to set an O2 sensor code but is off just enough to affect the accuracy of the catalyst monitor.



We had a Toyota Camry in a while ago with a new OEM cat and upstream O2 installed and still throwing PO420.
After replacing the downstream the code is gone for over a year now.
 
Did you look at the Mode $6 data before and after you replaced the downstream sensor? Look at the downstream switch rate with a scantool?
 
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