P0120
Throttle Position Sensor/Switch A Circuit Malfunction
Powertrain Fuel and Air Metering Throttle Control 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

The throttle position sensor tells your engine computer how much you're pressing the gas pedal, and it's sending a bad signal or no signal at all. It's like your car's ears not hearing your foot commands properly.

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Symptoms You May Notice
3 known symptoms for this code
Check Engine Light illuminated
Engine hesitation or surging at idle
Reduced fuel economy and poor acceleration
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECM monitors the throttle position sensor voltage output (typically 0.5V at idle to 4.5V at full throttle) to determine engine load and fuel injection timing. The sensor signal must follow a smooth linear pattern; erratic, stuck, or out-of-range voltages trigger this fault. The ECM compares sensor data against expected values based on pedal position inputs.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
TPS Voltage 0.5V (idle) to 4.5V (WOT) Below 0.5V or above 4.5V, or erratic fluctuation
Circuit Continuity Continuous signal path Open circuit, short to ground, or short to power
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
TPS connector and wiring
Inspect the connector at the throttle body for corrosion, moisture, or loose pins and clean or reseat as needed.
2
Throttle Position Sensor (TPS)
Disconnect the sensor and test continuity with a multimeter; replace if open or shorted.
3
Engine Control Module (ECM) software
Have dealer perform ECM reprogramming or reflash if wiring tests pass but fault persists.