Probable Causes:

WARNING: This page is about a different car, the 1993 Toyota Celica. However, it is still accessible from the selected car via links, so may be relevant.

A/C Switch or Switch Circuit
Accelerator Pedal or Cable
Neutral/Start Switch or Switch Circuit
Throttle Position Sensor IDL Circuit
ECM

To check throttle position sensor signal, see Code 41.

To check A/C switch signal, use the following procedure.

CODE 51/TEST 11-SWITCH SIGNAL TROUBLE

Terminal	Trouble	Condition	STD Voltage
A/C-E1	No Voltage	Air Conditioning ON	8-14V

Fig 1: Code 51/Test No. 11 Flow Chart - Switch Signal

Fig 2: Code 51/Test No. 11 Schematic - Switch Signal