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There is a sensor in the air intake stream called ACT (Air charge temperature). This sensor is what the code is referring to.
The failure indicates that either:
1. The sensor is giving a false high reading or (Unplugged, shorted, loss of power, etc.)
2. The sensor is defective, giving a reading outside of the expected normal range.
It's job is to measure the temp of the incomming air stream to the engine. Used together with the flow rate from the MAF sensor, the computer can calculate the density of the air charge. (hot air of the same volume versus cold air of the same volume, contain differing amounts of oxygen).
It may be located anywhere in the incomming air stream, but you may find it near the throttle body screwed into the upper intake plenum.
Thanks. The code came on after I replaced the power steering pump -- I had to remove the air filter and breather assembly to finish routing of the belt and in order to be able to apply the belt conditioner after re-installation.
Best thing to do then would be to take the 4.6L cover off and inspect for loose/unplugged connections?
Just did a quick check of my 99 4.6 and this sensor actually is located in the flex section of the air feed duct assembly, Immediately behind the MAF. Feel around for the plug and find the loose wire connection.
<---- feels like an idiot. Thanks Dialtone -- it was unplugged. On the backside of the flex ducting, before the assembly turns into the throttle body, right?