HI there
This code is for the TPS on the car, but it doesn't make any sense the issue is only happening in forward gears. I would check the engine mounting. If it's moving to much it could be causing a loss of connection at the TPS.
P0121 TOYOTA - Throttle/Pedal Position Sensor/Switch 'A' Circuit Range/Performance Read more: https://www.autocodes.com/p0121_toyota.html
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