OBD II scan tool (excluding hand-held tester):

WARNING: This page is about a different car, the 2004 Lexus IS 300. However, it is still accessible from the selected car via links, so may be relevant.

Check voltage between terminals VPA and E2, and VPA2 and E2 of ECM connector.
PREPARATION:
1. Remove the ECM hood (See ENGINE CONTROL MODULE (ECM) ).
2. Turn the ignition switch ON.
CHECK:

Measure the voltage between terminals VPA and E2, and VPA2 and E2 of the ECM connector.

Fig 1: Measuring Voltage Between Terminals VPA And E2, And VPA2 And E2 Of ECM Connector
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

OK:

Fig 2: Accelerator Pedal Voltage Chart
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
1. OK: Go to step 3 .
2. NG: Go to next step.
Check for open and short in harness and connector between accelerator pedal position sensor and ECM (See HOW TO USE THE DIAGNOSTIC CHART AND INSPECTION PROCEDURE ).
1. NG: Repair or replace harness or connector.
2. OK: Replace accelerator pedal position sensor (See REPLACEMENT ).
Is the DTC P2121 being output again?
PREPARATION:
1. Clear the DTC (See PRE-CHECK ).
2. Start the engine.
3. Run the engine at idle for 15 seconds or more.
CHECK:
1. Read the DTC (See PRE-CHECK ).
Result:

Fig 3: Display (DTC Output) Chart
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
1. B: System OK
2. A: Replace ECM (See HOW TO USE THE DIAGNOSTIC CHART AND INSPECTION PROCEDURE ).