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Old November 22nd, 2018, 10:17
lawsoncl lawsoncl is offline
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Join Date: Jan 2006
Location: North Idaho
Posts: 5,311
Re: Sudden drop in fuel economy

I would suspect a 99 XJ would throw a code for a bad O2 sensor, but possibly not. If memory serves, the CTS on a 99 is the one in the thermostat housing. Resistance should be pretty high when cold, 5-10kohms. Once warmed up, it should be fairly low, around 100-300 ohms or so. When they go bad they often just open up and show really high resistance or open circuit and don't change.
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