On the 1996 Cadillac DeVille, when P0171 and P0174 are set with oxygen sensor codes, more times than not, this is not an oxygen sensor problem. This means the air-fuel mixture is running “lean” and this is what sets the oxygen sensor code. It’s not very common for both upstream oxygen sensors to fail at the same time. So, before condemning the oxygen sensors – First- check the Fuel Filter. Remove the filter and try to blow through it. Any resistance and the Fuel Filter will need to be replaced. If the fuel filter is ok, a fuel pressure reading should be taken to ensure the Fuel Pump is working at normal capacity. If these check ok, next move onto the air filter housing, duct and filter element. Inspect the duct for any cracks, holes or loose connections. Make sure the filter box is sealed tightly and correctly. Inspect the filter element and make sure it is not excessively dirty, restricting air flow. With the engine running, listen for any “sucking” or “hissing” noises. Inspect as many vacuum lines on the top and rear of the engine as you can see, looking for any cracks, holes or loose/missing connections. Make sure there are no leaks in the exhaust system. If these check ok, an intake air leak should be done to make sure there are no leaks. With the engine running, carefully and sparingly spray some throttle cleaner around the base of the intake. If the engine responds, this is a sign of a leak and the intake gasket will need to be replaced (make sure to visual inspect the manifold itself for any cracks at this time as well). If these all check ok, then the problem will lie in the Mass Air Flow Sensor. Inspect the electrical connector for any loose, broken or damaged wires. If the visual inspection checks ok, then there IS a “cleaner” for the MAF sensor, however, in experience, it’s usually not a 100% fix, and when it does work, it’s usually not for long, and MAF replacement is usually necessary.
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