OBDII PO420 CEL Code Help - Team Camaro Tech
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post #1 of 16 (permalink) Old Sep 9th, 11, 12:48 PM Thread Starter
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Al
 
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OBDII PO420 CEL Code Help

Hi guys -

This is on my 98 Dakota p/u, 3.9 V6, AT, 62,000 miles. Code PO420 "Catalyst System efficiency below threshold bank 1".

I just replaced the cat last year with a Magnaflow replacement so I hope it's not that. No CEL then, but the guts had broken loose and were rattling. 6-7,000 miles on new cat. I cleared the code with a scan tool and it came back after about 200 miles.

Any experience, insight or help greatly appreciated.


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post #2 of 16 (permalink) Old Sep 9th, 11, 03:05 PM
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Re: OBDII PO420 CEL Code Help

Could be a bad Oxygen sensor.. Iv noticed when a cat goes most of the time the o2 sensor goes..

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post #3 of 16 (permalink) Old Sep 9th, 11, 04:36 PM
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Re: OBDII PO420 CEL Code Help

I was gonna say the same thing. Depending on the vehicle and model, there may be two O2 sensors per bank also.
I found that out when I started getting a 0430 (bank 2) CODE.

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post #4 of 16 (permalink) Old Sep 9th, 11, 04:55 PM
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Re: OBDII PO420 CEL Code Help

As suggested, could be the second O2 sensor after the cat.
A cheap fix, at least for older Honda's, 95 to 2005? - buy a couple spark plug anti-foulers - remember those?
Drill out the hole of the first one to clear the O2 sensor bulb, stack on the second one, then reinstall sensor with foulers and plug-in, clear code and drive away.

At least it worked on my '98 Civic DL w'1.6L.

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post #5 of 16 (permalink) Old Sep 10th, 11, 11:11 AM Thread Starter
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Re: OBDII PO420 CEL Code Help

Thanks for the help guys. I cleared the code again, then drove around with the code reader attached until the computer had run all of its tests and the emissions light on the tester went from yellow to green - hustled over to the state state testing station and passed. Good to go for 2 more years.


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post #6 of 16 (permalink) Old Sep 10th, 11, 02:38 PM
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Re: OBDII PO420 CEL Code Help

That's what I do.... clear the code the night before state inspection.

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post #7 of 16 (permalink) Old Sep 10th, 11, 02:51 PM Thread Starter
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Re: OBDII PO420 CEL Code Help

Yeah - I found out that once you clear a code that the computer has to do its self-diagnostics on everything it monitors again. The code reader I have to use shows which diagnostics have and haven't been completed by flashing a little icon on the screen. The last 2 to complete for mine were the heated o2 sensor and the catalyst. The o2 light quit flashing on the second day. On the third day I took it out for a freeway drive and the catalyst check completed, so I scurried over to emissions testing. I fully expect the CEL to come back on....


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post #8 of 16 (permalink) Old Sep 10th, 11, 05:05 PM
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Re: OBDII PO420 CEL Code Help


Mines been on for over a year... I just clear it for inspections. Then it comes back on in a couple days.

But I haven't driven the truck except for a few miles all summer and just recently started driving it the other day.... the light goes out by itself and hasn't come back on yet!
Figure that one out!

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post #9 of 16 (permalink) Old Sep 11th, 11, 04:02 AM
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Re: OBDII PO420 CEL Code Help

Just went thru a 0420 on a chevy venture. Had a Engine Coolant Temp sensor reading 235 with the car cold. After we changed that the code cleared. Wish We had found it before we replaced all the plugs, wires and two 02 sensors.

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post #10 of 16 (permalink) Old Sep 11th, 11, 04:19 PM Thread Starter
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Re: OBDII PO420 CEL Code Help

Quote:
Originally Posted by yellow69RS View Post
Just went thru a 0420 on a chevy venture. Had a Engine Coolant Temp sensor reading 235 with the car cold. After we changed that the code cleared. Wish We had found it before we replaced all the plugs, wires and two 02 sensors.

Jeff
How do you test that sensor, Jeff? I assume that it's a separate sensor than the one for the gauge.


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post #11 of 16 (permalink) Old Sep 11th, 11, 04:26 PM
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Re: OBDII PO420 CEL Code Help

Yes, it is. If he used a scanner, it would have told him the engine coolant temp.
Once a high temp is indicated, he would have pulled the two-wire connector from the sensor and measure across the the sensor pins and read a low ohmic value.
Generally, anything above 8K ohms is cold.

Or, the code reader would have given the fault code for a faulty coolant sensor and troubleshoot the same from there.

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post #12 of 16 (permalink) Old Sep 24th, 11, 07:53 AM
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Re: OBDII PO420 CEL Code Help

I had to get the cats replace on my 02Z. I was surprised to see 420 come back just a few days after the replacement. I cleared the code and they have yet to return. I wonder if the new cats had to break-in. No 420/430 codes since.

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post #13 of 16 (permalink) Old Sep 25th, 11, 07:22 AM
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Re: OBDII PO420 CEL Code Help

Quote:
Originally Posted by BPOS View Post
How do you test that sensor, Jeff? I assume that it's a separate sensor than the one for the gauge.
Another Mechanic with a scan tool read the temp after the car had sat 3 hours. I took his advice to change it, and yes it's separate from the guage sender as the guage still reads that the car is running hot. I just replaced the water pump in this thing a few days ago. (ARRRGH!)

Jeff
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post #14 of 16 (permalink) Old Dec 13th, 11, 11:09 AM Thread Starter
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Re: OBDII PO420 CEL Code Help

Hi guys - bringing this back up. To refresh - 98 Dakota, 3.9 V6, at, 2wd, 64k miles. Cat replaced last year. Still getting the po420 cat efficiency code, but the last CEL was a po301 misfire on cyl #1. Do they seem related? If a cyl is misfiring, could that cause the computer to think it's a bad cat? Thanks again!


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post #15 of 16 (permalink) Old Jan 3rd, 12, 06:29 AM
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Re: OBDII PO420 CEL Code Help

Quote:
Originally Posted by BPOS View Post
Hi guys - bringing this back up. To refresh - 98 Dakota, 3.9 V6, at, 2wd, 64k miles. Cat replaced last year. Still getting the po420 cat efficiency code, but the last CEL was a po301 misfire on cyl #1. Do they seem related? If a cyl is misfiring, could that cause the computer to think it's a bad cat? Thanks again!
Depends on what is causing the misfire. A lean or rich condition causing the misfire can factor into the 0420. Also, the misfire can cause a lean/rich condition to cause the 0420 code. running too rich or, too lean can damage the cat. The 0420 code is set when the waveforms of before and after cat 02 sensors are too similar indicating that the cat is not affecting the exhaust. Find and fis the misfire code first.
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