Fix P0120: Mechanic’s solutions that WORKED

The code P0120 points out a fault in the Throttle/Pedal Position Sensor ‘A’ Circuit, which plays a critical role in managing the engine’s air intake and acceleration. Most likely, this issue is caused by a faulty sensor, wiring issues, or problems within the throttle body system. Addressing this code is essential for maintaining the vehicle’s proper acceleration and performance.

P0120 quick overview

Meaning	P0120: Throttle/Pedal Position Sensor/Switch ‘A’ Circuit Malfunction
Is it serious?	This code can be serious as it relates to the vehicle’s throttle system, potentially affecting the car’s acceleration and overall performance.
Possible causes	– Faulty Throttle Position Sensor (TPS) – Damaged or disconnected wiring to the TPS – Corrosion or poor connections in the TPS circuit – Faulty Accelerator Pedal Position Sensor – Problems with the throttle body
How to diagnose?	Check TPS and Wiring: Inspect the Throttle Position Sensor and its wiring for damage or disconnection. Test TPS Function: Use diagnostic tools to test the functionality and response of the TPS. Inspect Connectors: Examine connectors for corrosion or poor connections. Evaluate Pedal Position Sensor: Check the Accelerator Pedal Position Sensor, if applicable. Clean Throttle Body: Inspect and clean the throttle body to ensure proper operation. Check for Ground Issues: Look for grounding problems in the circuit connected to the TPS.

How to fix P0120 – learn from mechanics

I looked into P0120 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Mitubishi

Vehicle Information: 2000 Mitsubishi Mirage DE 1.8L

Issue + Repair History:

• MIL lamp on with code PO100 for mass air flow meter.
• Pin tested connector at meter, powers and grounds were good.
• Original mass air flow meter had a bad pattern on scope testing.
• Replaced meter with OEM unit, but the same code returned.
• Scoped new meter, pattern was good.
• Checked at PCM, pattern was also good, but set a PO120 code.
• Replaced PCM, but the same codes returned.

Computer Codes & Descriptions:

• PO100: Mass Air Flow Sensor.
• PO120: TPS Sensor.

Suggestions to Fix Issue:

• Perform a voltage drop test throughout the wiring.
• Clean the throttle body and ensure proper spark plugs (NGK) are used.
• Check distributor cap for corrosion or rust on the coil terminal.
• Inspect the hose between the MAF and Throttle Body for cracks.

Final Fix:

• Found TPS sensor had an intermittent short to ground, affecting the voltage supply to the mass air flow meter.
• Replaced TPS sensor, resolving the issue with no further problems.

Case #2: Mercury

Vehicle Information: 1996 Mercury Villager GS 3.0L

Issue + Repair History:

• Van came in with two codes: PO120 and PO325.
• Code PO120 could not be found; similar to PO121.
• After clearing codes and driving for about 25 miles, only code PO325 returned, but MIL light stayed off.
• Van had never been tuned up, with all original equipment.
• Question raised about whether the computer was correctly processing the knock sensor signal.

Computer Codes & Descriptions:

• PO325: Knock Sensor 1 Circuit Malfunction.
• PO120: TPS Sensor Circuit Malfunction.

Suggestions to Fix Issue:

• Replace the TP sensor, as it was likely faulty.
• Set the TP sensor voltage to 0.46 at idle.
• The knock sensor circuit is constantly monitored, requiring the fault to be detected 3 consecutive times under the same driving conditions before turning the MIL on.
• Suggestion to replace the knock sensor if unsure about how to check the circuit components.

Final Fix:

• Replaced both the TPS sensor and knock sensor.
• After the replacements, no more codes were reported and the issue was resolved.

Case #3: Mercedes

Vehicle Information: 2001 Mercedes-Benz S500 Guard 5.0L

Issue + Repair History:

• Car was in a body shop for side window replacement and door panel work.
• After receiving the car, the throttle would not respond.
• Cleared PO120 code, but the issue persisted.

Computer Codes & Descriptions:

• PO120: Throttle Sensor A Circuit Malfunction.

Suggestions to Fix Issue:

• Check wiring to the throttle valve actuator.
• Possible replacement of the throttle valve actuator.
• Suggestion to replace the gas pedal assembly as it might contain a faulty position sensor.
• Perform a relearn procedure: turn on the ignition, wait 45 seconds, start the engine, let it idle for 3 minutes.
• Check wires from the accelerator pedal to the engine control module.
• Consider replacing the accelerator pedal or throttle actuator.

Final Fix:

• The issue was resolved by replacing the accelerator pedal.

Case #4: Lexus

Vehicle Information: 2000 Lexus GS300 3.0L

Issue + Repair History:

• Plan to replace the adjustable TPS sensor.
• Modus shows TPS % but not voltage.
• Needed guidance on adjustment procedure and voltage specifications.

Computer Codes & Descriptions:

• PO121: TPS Out of Range.
• P0120

Suggestions to Fix Issue:

• Look under PO120 TPS Circuit Malfunction in ShopKey for closed voltage and WOT volts specifications.
• TPS voltage should be 0.4-1.0 volts when throttle is closed and 3.2-4.8 volts at WOT (Wide Open Throttle).
• Adjust the TPS while idling and then drive the vehicle with data stream, stopping to make adjustments.
• Adjust the TPS so the CTP (Closed Throttle Position) just closes with the throttle on the stop.

Final Fix:

• Referred to the ShopKey under a different code (PO120) for starting voltage points.
• Recorded voltages on the defective TPS to get enough information to install and adjust the new TPS.
• The new TPS installation and adjustment resulted in the car running great.

Case #5: Toyota

Vehicle Information: 2000 Toyota Tundra SR5 4.7L

Issue + Repair History:

• After stopping at a light, the truck remained at idle despite pressing the gas pedal.
• Truck towed to the shop, functioning normally upon arrival.
• Codes P0120 and P0121 were set.

Computer Codes & Descriptions:

• P0120: TPS/Pedal Position Sensor A Circuit Malfunction.
• P0121: TPS/Pedal Position Sensor A Circuit Range/Performance.

Suggestions to Fix Issue:

• Test both the foot feed TPS and the throttle body TPS for glitches.
• Check connections for contamination.
• Conduct a warm engine, KOEO sweep of the TPS.
• Remove and clean the connector on the accelerator pedal position sensor.
• Consider the possibility of Pedal Position Sensor or Throttle Control Motor failure.
• Check the throttle body for sticking.
• Inspect the TPS sensor in both hot and cold conditions.

Final Fix:

• Cleaned harness connector terminals at TPS.
• Drove the vehicle for three days and then returned it to the customer, issue resolved.

Advice	Brief Description of Diagnostic Step
Check and Clean TPS Connections	Inspect the Throttle Position Sensor’s connectors for contamination and clean them.
Test TPS Function	Perform tests on the Throttle Position Sensor, both at idle and when the throttle is engaged.
Check Wiring and Connectors	Inspect the wiring and connectors leading to and from the TPS for any damage or loose connections.
Consider Sensor Replacement	If testing indicates a fault, consider replacing the Throttle Position Sensor.
Check for Ground Issues	Look for any grounding problems in the circuit connected to the TPS.
Inspect Accelerator Pedal Position Sensor	Examine the sensor at the pedal, which could be the source of the problem.
Perform a Throttle Sweep Test	Conduct a test with the engine off (KOEO) to monitor TPS behavior throughout the throttle range.
Adjust Sensor if Necessary	Make adjustments to the TPS as per the manufacturer’s specifications if it’s adjustable.
Clean Throttle Body	Inspect and clean the throttle body to ensure the throttle plate is not sticking.