Check Engine Light Due to Gas Cap or Catalytic Converter on Cadillac DeVille

Two likely causes for the Check Engine Light to illuminate would be a loose or worn gas cap or a faulty catalytic converter.

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Average mileage: 107,166 (55,000–160,000)
4 model years affected: 1998, 1999, 2001, 2004
11 people reported this problem
5 people shared problem details
1999 Cadillac DeVille116,000
check engine light on ... going to put new catalytic converter on Friday will let u know if it fixes th problem
Check engine light comes on, check gas cap warning. Mine is caused by a rotted gas overflow line. Seems that it has rotted so bad that when I top off the gas tank, it started overflowing to the ground. It is also causing a miss in my engine when accelerating (not getting full power due to fuel pressure). Have not repaired yet but have found the fuel filler neck on eBay for $70.00. I was told you can cut off the bad section of the overflow, and clamp on a 3/4 ID fuel line. I am certain this would work, however if the line is that rotted I think I will install a new one for the $70 price.
Intermittant engine light due to gas filler cap. Replaced cap, but intermittant problems persists. No fix yet.
1998 Cadillac DeVille160,000
I keep getting MIl Code P0440, and have repaired everything in evap system w/o curing this problem.
check engine light comes on when gas reading reaches 8, 10 and 11 gals. Had new catalytic converter put on also o2 sensor thought everything would be alright, but when gas reading came down to 11 gals. check engine light came on again, had it check by computer and got the same reading as befor bad 02 sensor.
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