Troubleshooting

DTC P0121 THROTTLE/PEDAL POSITION/SWITCH "A" CIRCUIT RANGE/PERFORMANCE PROBLEM

CIRCUIT DESCRIPTION




The throttle position sensor is mounted in the throttle body and detects the throttle valve opening angle. When the throttle valve is fully closed, a voltage of approximately 0.3 to 0.8 V is applied to terminal VTA of the ECM. The voltage applied to terminal VTA increases in proportion to the opening angle of the throttle valve and becomes approximately 3.2 to 4.9 V when the throttle valve is fully opened. The ECM judges the vehicle driving conditions from this signal input from terminal VTA, and uses it as one of the conditions for controlling the air-fuel ratio, power increase and fuel-cut.

DTC Detecting Condition:

The throttle position sensor is mounted on the throttle body, and has 2 built-in sensors to detect the throttle value opening angle and malfunctions of the throttle position sensor.

The voltage applied to terminals VTA and VTA2 of the ECM changes between 0 V and 5 V in proportion to the opening angle of the throttle valve. The VTA is a signal to indicate the actual throttle valve opening angle, which is used for the engine control. The VTA2 is a signal to indicate the information about the opening value angle, which is used for detecting a malfunction.

The ECM judges the current opening angle of the throttle valve from these signals input from terminals VTA and VTA2, and the ECM controls the throttle motor to make the throttle valve angle properly in response to the driving condition.

If this DTC is stored, the ECM shuts down the power for the throttle motor and the electromagnetic clutch, and the throttle valve is fully closed by the return spring.

However, the opening angle of the throttle valve can still be controlled by the accelerator pedal through the throttle cable.

MONITOR DESCRIPTION

Monitor Strategy (M/T):

Typical Enabling Condition (M/T):

Typical Malfunction Thresholds (M/T):

Component Operating Range:

M/T:
The throttle position sensor varies its resistance with the angle of the throttle valve. The ECM applies a regulated reference voltage to the throttle position sensor (+) terminal and calculates the angle of the throttle valve based on the current voltage at the throttle position sensor signal terminal.

When the throttle valve is near the fully closed position, the output voltage of the throttle position sensor is low. When it is near the fully open position, the output voltage is high.

The ECM checks the indicated angle of the throttle valve during stop and go" conditions. If the indicated angle (or voltage) in the closed throttle position is out of the specified range 8 times or more, the ECM interprets this as a malfunction in the throttle position sensor and sets a DTC.

Monitor Strategy (SMT):

Typical Enabling Condition (SMT):

Typical Malfunction Thresholds (SMT):

Component Operating Range:

SMT:
The ECM uses the throttle position sensor to monitor the throttle opening angle.

This sensor including two signals, VTA and VTA2. VTA is used to detect the throttle opening angle and VTA2 is used to detect malfunction in VTA. There are several checks that the ECM performs confirm proper operation of the throttle position sensor and VTA.

There is a specific voltage difference expected between VTA and VTA2 for each throttle opening angle. If the voltage output difference of the VTA and VTA2 deviates from the normal operating range, the ECM interprets this as a malfunction of the throttle position sensor. The ECM will illuminate the MIL and set a DTC.

FAIL SAFE

SMT:
ECM cuts off throttle actuator current and throttle valve controlled by throttle cable.

ECM then adjusts engine output by controlling fuel injection (intermittent fuel-cut) and ignition timing in accordance with accelerator pedal opening angle so that the vehicle can continue to be driven at a minimal speed.

Step 1:

INSPECTION PROCEDURE

HINT: Read freeze frame data using the hand-held tester or the OBD II scan tool. Freeze frame data records the engine conditions when a malfunction is detected. When troubleshooting, freeze frame data can help determine if the vehicle was running or stopped, if the engine was warmed up or not, if the air-fuel ratio was lean or rich, as well as other data from the time when a malfunction occurred.