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I'm getting CEL code 43 after doing a lot of work on my car (changing the head gasket, which involved removing and reattaching lots of exhaust and intake components, and replacing the fuel filter and other general maintenance), but otherwise it seems to be running fine. According to the Haynes manual this code means "fuel supply system oxygen sensor" and has some references for sections of the manual to see, but I can't find anything about what specifically would cause the CEL.

In case it's relevant, I also have the cooling fan wired to always-on, because the electronic sensor that switches it on/off is malfunctioning and never turning it on, so it takes the engine a bit longer than usual to get up to normal operating temperature. But I've run with no the thermostat removed before and never experienced a CEL 43, so I doubt this is the issue.

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up vote 3 down vote accepted

Replacing the broken fan switch seems to have made the CEL go away. My best guess is that the ECU starts monitoring the O2 sensor output as soon as this sensor reads that the engine is hot, and gets upset when the O2 sensor is actually still cold.

Edit: Nope, the CEL is back, and gas mileage has been really bad (around 18-19mpg on a car that usually gets 28-30 city and 40+ highway). Any better ideas?

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