PO420, but only above 1/2 tank... O_o

coondogg

coondogg

New member
Since my last trip into the garage, I've been watching my check engine light (po420) spending long periods on and off. Honestly, I didn't recognize any differences in my driving style so I was trying to see if there was any pattern and what was causing the cel to go on and off like that. As an attempt to address the light for the po420, I picked up those spark plug arrestors and still plan on installing them on the back o2 as a possible "fix". Almost 800ish miles later and I realized that the light is only on till my tank gets to ~1/2 level then it goes out until I fill the tank again. I've been able to confirm this theory over my last 3 tanks of gas.

Any clue as to why the fuel level would effect a catalyst efficiency code? I'm ruling out an issues with the fuel or the tank its self due to the length off time burning up each tank is not consistent but possible fuel filter? Well, I'm going to replace the fuel filter anyways just for maintenance since I have no clue when it was last done and it looks easy as shiet to do.

I hate this code because there are so many expensive parts that could be the real problem and I don't want to blindly throw parts at it and not repair the issue.

 
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the 420 code can, I believe, also come on for the gas cap. Check/replace your gas cap, and check your fuel filler neck isn't leaking.

If it was something to do with the gas cap or filler, fuel level would make sense.

Also, that code is just for the rear O2. The rear O2 doesn't really do anything besides tell the computer if the cat is working for emissions purpose.

My leagcy is completely missing the rear O2, someone must have installed a resistor or something at some point because I don't have a CEL for it, and the sensor is missing.

 
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Hmm, not finding anything on NASIOC (not that you can actually find what you're looking for with their search) with gas cap and po420's...

987687 said:
Also, that code is just for the rear O2. The rear O2 doesn't really do anything besides tell the computer if the cat is working for emissions purpose.
Click to expand...
That was what I found when researching the code. Really threw me of a loop when the gas level was effecting when the CEL would come on!

 
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For $5, figured that it was worth a try since the shop reported to me that the rear o2 was doing its job when they were looking at it.

 
It's either a bad o2 sensor, bad convertor or an exhaust leak somewhere near the cat. The flanges are known to rot away and expose the gasket material which will cause the code.

 
The actual light comes on and off depending on fuel level? But throws the same p0420 code? No other codes to go with it. Nothing pending.

 
Yup, just had it read again yesterday and it was only the po420. It was suggested to me on NASIOC to just fill it to 1/2 a tank and see what it does so I'm trying that now. I just put 5 gallons in it which put it up to 1/2 a tank. I reset my trip B odometer so I can see exactly how many miles it will go before the cel pops again (if it even does) and going from there. Have gone 10 miles so far and no cel.

 
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Theory is bonked. After 23 miles, cel came back on. Filled it up with 91 this time and will watch my mileage as to when/if it goes back off...

 
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