On the 2007 Cadillac Escalade, the P0121 is pointing to a problem with the Throttle Position Sensor. Inspect thoroughly for any type of leak or restriction at the throttle body and throttle plate. Check for air leaks at throttle body mounting area and intake manifold sealing surfaces. Inspect throttle body for dirt, debris, and coking. If any doubts, a good throttle body cleaning can sometimes cure this code. With the use of a toothbrush and some throttle cleaner, get as much build up off the throttle bore and plate as you can. If the visual inspection checks ok, and the cleaning did not solve the problem, then the Throttle Body will need to be replaced, as the Throttle Position Sensor is built into the throttle body and not serviceable as a stand-alone item.
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