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------------------------------------------------ The following statements are all correct EXCEPT:

A. Relearning the PCM should never be attempted.
B. On engines operating very poorly, the PCM should be relearned after fuel system cleaning.
C. Fuel systems that have never been cleaned may have developed wax deposits as well as carbon.
D. The fuel rail and pressure regulator should be cleaned as well as fuel injectors.

In the context of automotive technology, relearning the PCM is sometimes necessary, particularly after cleaning a poorly operating fuel system, and both the fuel injectors and delivery system components should be maintained to avoid deposit accumulation.

Explanation:

The subject of this question is related to Automotive Technology, specifically focusing on the Powertrain Control Module (PCM) and fuel system maintenance. The correct answer to this question is that statement a is the incorrect one. Relearning the PCM can be necessary in certain conditions. For instance, after fuel system cleaning, especially if the engine was operating very poorly prior to the service, relearning the PCM can help the vehicle's computer system recalibrate for the changes in engine behavior post-cleanup. One might also need to clean both the fuel injectors and the components of the fuel delivery system such as the fuel rail and pressure regulator to remove any build-up of deposits, including wax and carbon, particularly if the fuel system has been neglected over time.