P0121 is a TPS sensor performance code. (Throttle Position Sensor #1)
When Monitored: With engine running and no MAP sensor or TP sensor DTC's set.
Set Condition: The PCM determines a valid range in which the TP sensor should be, at a given RPM/Load. The actual TP sensor voltage is then compared to this value. If the TP sensor voltage does not fall within the expected range within a predetermined time an error will be detected. One trip Fault.
My Commentary (random musings) - With the history of the P1188 that you have had - with no modifications, and the term LOAD used in the above Set Condition description I am betting the 2 are related, and it has to do with the vacuum harness going to the solenoid pack on the passenger frewall or it's associated wiring.
I believe that the PCM uses vacuum as data to determine load. The P1188 error always seems to occur at the point that the TIP sensor is switched from measuring vacuum or pressure to determine or confirm the the state of the turbo. I would wager a guess that the ground integrity of the wiring to one of the solenoids or to the TIP signal at the PCM is compromised. If there is any difference between ground potential at the solenoid and where the data is referenced to ground, then a timing issue arises. If the data is delayed, then the corresponding action called by the PCM is occuring too late and sets the dominos in motion.
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