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question... please help

erik671

Member
Joined
May 10, 2001
Messages
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City, State
Pepperell, MA
Year, Model & Trim Level
'96 XLT
In the past with my 96 explorer, I have had the check engine light on, and it was the code for low catalyst efficiency. I replaced the cat & O2 sensors and the light was out for a while. It will now came on when I almost ran my tank empty (same code). I have heard that a problem with the gas cap can cause the light to come on. So, as a test, I drove 80 miles to work today without my gas cap on and the light never came on.

Could there be a problem with the gas cap where not having it on is better? Can a gas cap problem cause a low catalyst efficiency code on a 96 explorer?

The car runs fine, but this light is driving me MAD!!!:confused:

Please help.....

Thanks,
Erik
 



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From what I can remember from talking to the diagnostic tech at work that code might have something to do with the EGR valve... but don;t quote me on it. I'll chat with him tomorrow about it.

If worst some to worst, black electrical tape works wonders! ;)
 






Originally posted by erik671
In the past with my 96 explorer, I have had the check engine light on, and it was the code for low catalyst efficiency. I replaced the cat & O2 sensors and the light was out for a while. It will now came on when I almost ran my tank empty (same code). I have heard that a problem with the gas cap can cause the light to come on. So, as a test, I drove 80 miles to work today without my gas cap on and the light never came on.

Could there be a problem with the gas cap where not having it on is better? Can a gas cap problem cause a low catalyst efficiency code on a 96 explorer?

The car runs fine, but this light is driving me MAD!!!:confused:

Please help.....

Thanks,
Erik

The short answer is no.

The long answer is that when your gas cap fails it will set a P0455 code (gross leak detected in fuel system). It does not set a cat efficiency code (can't remember the number off hand). For this code to get set, your computer has to see the failure twice in a row after two successive cold starts (min. 8 hours in between). What happens is once your vehicle gets to the proper conditions to run the test, a vacuum will be applied to your fuel lines. If after a pre-determined amount of time the vacuum has dropped below a certain value, it will set the P0455 code in your Pending Codes. If it happens again after the next cold start, it will move the code to your Trouble Codes and set your CEL. The only way to know for sure why your CEL came on this time is to read the codes again and see what comes up.

A cat efficiency code gets set any time your upstream and downstream switch ratio gets above about 90%. Your PCM monitors the interval that your O2 sensors switch from lean to rich and vise versa. Your upstream sensors are supposed to switch at least every 3 seconds. Your downstream sensors should switch at a longer interval. Your PCM will determine the ratio. If the ratio gets too close to 1:1 it means that your catalytic converter is not doing its job and sets the cat efficiency code. A bad gas cap won't have anything to do with the code.
 






Thank you for the info.

With that in mind, could a failed upstream sensor that is not switching cause the cat. efficiency code? I am getting the code P0420 which is low cat efficiency bank 1. I have changed the cat and the downstream sensor, so could the upstream sensor that bank 1 refers to be the problem?

Thanks,
Erik
 






Don't know for sure, but I suppose it's possible. I would think you would get another O2 related code though if your upstream sensor failed to where it wasn't switching.
 






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