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Nick, Check the thermostat in the airbox, it regulates a flap that will open or close the pre-heat inlet from the exhaust manifold. If it's defective you suck hot air into the engine and drive up the N/O reading. All of my 240's have the thermstat,the flap, and pre-heater hose removed, so I pull in cold air only. I also cap-off the pre-heater inlet. The cars suffer no cold start problems at all, the pre-heat equipment is to reduce cold start emissions. Check it out, this could be your problem.
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Bruce S. near D.C.
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