Not Using SUZUKI Scan Tool

Remove ECM referring to Engine Control Module (ECM) Removal and Installation .
Connect couplers to ECM.
Using voltmeter, check voltage at "C46-22" terminal under the following each condition.
If measured value is out of specified range, check TP sensor referring to Step 12 of DTC P0121: THROTTLE POSITION SENSOR CIRCUIT RANGE/PERFORMANCE .

If OK, check wire harness of TP sensor.

Also check that voltage varies according to throttle valve opening linearly. If not, it is possible that TP sensor has failed. Replace.

TP sensor voltage

When throttle is fully close: 0.70 +/- 0.15 V

When throttle is fully open: 4.0 +/- 0.5 V

Fig 1: Checking TP Sensor Voltage Between Terminals
Courtesy of SUZUKI OF AMERICA CORP.
Upon completion of checking, install ECM.