Question about 1995 Cadillac DeVille

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95 caddlilac seville STS getting horrible fuel economy.

I replaced all the two O2 sensors a year ago, and I just replaced the catalytic converter in Oct of 2008, the plugs and wires are only a year old and I've replaced the PCV valve. Now I'm getting a check engine light with the trouble code P042 Left Oxygen Sensor Signal Lean. I use to get on average about 20mpg, not bad for a car with 435,000km on the original drivetrain. Now I'm lucky if I get 12mpg. I also had to change my oil again today as it smelled like raw fuel, the oil that was in it only had only about 20% use on it. I did not want to gas wash the engine so I just replaced it to be safe. Can this major change in fuel economy be caused by a small vacume leak or is just the O2 sensor has prematurally bit the dust due to its high mileage engine?
Any help or suggestions would be greatly appreciated.

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You will see shorter service life of those O2 sensors with that kind of mileage. I would also suspect that the injectors are seriously worn if original. a few things to also look at that tend to throw 02 codes:

1: MAP OR MAS sensors,
2. worn timing chain
3: Restricted air intake (filter, or leak between MAS sensor and Intake)
4: Engine coolant thermostat (if the engine never warms fully, the mixture will be overly rich)

The O2 sensors tend to get wiped out by too much or too little fuel, too much in your case. you have to find the problem of fuel delivery or it will kill any O2 sensors you replace.

Have you performed a compression check? in some cases the piston, or rings have failed and allowed a large amount of fuel to enter the crankcase oil.

Posted on Jan 04, 2009


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I would recommend replacing the O2 sensor then go from there. If it returns then you will have to start testing the wiring. I would recommend changing the O2 sensor now if you are able to.

It is simple to replace and easy to get to. The trip would be a good chance after the repair to see if it returns with the number of miles you would be driving.


1. Oxigen Sensoir connector
2. Catalytic Converter
3- Downstream Oxygen Sensor
4. Engine Harness connector.

It can have an effect on engine performance and fuel economy. So the sooner you replace it the better. I dont believe it will leave you stranded on the road anyway by any means so I wouldnt worry about that if you arent able to replace it right away before the trip. If you need more help or have more questions then let me know and I will help you further.

Hope heplp with this (remember rated this post).

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