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P0121 OBD Fault Code (Causes & Fixes)

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Last Updated on: 7th September 2023, 12:36 am

The P0121 fault code stands for: Throttle Position Sensor (TPS) “A” Circuit Range Performance Problem

It’s triggered when the ECM detects that the throttle position “A” circuit’s output voltage has gone above or below the expected range.

P0121 is a sign of a severe problem and may cause a breakdown, therefore you should avoid driving the vehicle if this fault code appears.

In this article, we’ll explain what the P0121 OBD fault code means, its signs and causes, and how to fix it.

What Does P0121 Mean?

The code P0121 corresponds to the throttle position sensor (TPS) “A” circuit range performance issue.

It comes into play when the Engine Control Module (ECM) perceives that the output voltage of your throttle position “A” circuit has deviated from the projected range, either exceeding it or falling below it.

Similar to the TPS “B” Circuit, the TPS “A” Circuit has an expected output voltage range.

The emergence of this fault code indicates that the throttle position “A” circuit’s output voltage has gone beyond the range specified for this particular sensor.

How Serious Is It?

When the P0121 code is triggered, your vehicle might engage failsafe mode, commonly known as limp mode.

Limp mode will reduce the power output of the engine and usually limits the RPM the engine will reach, reducing acceleration and power.

This problem has the potential to lead to stalling, power loss, and harsh running of the engine. In some cases, the vehicle may have difficulty starting or running.

Signs & Symptoms

There are various signs of a P0121 OBD fault code.

You may notice the following signs:

  • The check engine light on the dashboard
  • Jerking upon acceleration
  • Reduced acceleration and power
  • Engine stalling
  • Rough running engine and poor idle

Possible Causes

There are multiple potential causes for a P0121 code, the following are the most common causes.

  • An open or short of the TPS circuit
  • Corrosion or damage to the wiring or connectors
  • Dirty or faulty throttle body
  • Loose connection of the TPS connector
  • Malfunctioning throttle position sensor (TPS)

How to Fix

Make sure to verify the P0121 code is the underlying reason for the issues by using an OBD scanner tool. Once verified, follow the steps detailed below.

You may the following tools:

  • a screwdriver for any clamps
  • a socket or ratchet set
  • a cloth
  • throttle body cleaner
  • an OBD scanner

Many vehicles with 80,000-100,000 or more miles have sensor faults that arise while starting up or during sustained driving.

  1. Check if there are technical service bulletins (TSBs) available for your vehicle to see if there is a known problem that may help you in fixing the issue.
  2. Scan the OBD system to check for other fault codes or issues. If any of these are present, they should be investigated also.
  3. Clear the error codes, start the vehicle and test drive it in a safe area (if no serious issues are present) while a diagnostic tool scans the live data.
  4. If the P0121 code is still present, examine the wiring for damaged or loose connections. If any issues are found, repair as needed.
  5. Clean the throttle body. If this does not resolve the issue you may need to replace the throttle body.

Cleaning the Throttle Body

Take the intake pipe out of the throttle body, spray throttle body cleaner onto a clean cloth.

For vehicles with a cable-operated throttle, you can manually open the throttle by pulling the cable on the throttle body’s side or pressing one side of the throttle plate.

Gently remove carbon buildup from both the throttle plate and the throttle body until they are thoroughly clean.

Following the throttle body cleaning, it might be necessary to perform a throttle body relearning procedure, especially in modern vehicles equipped with electronic throttle bodies.

Tip: Apply throttle body cleaner onto a cloth and then wipe, avoid directly spraying into the throttle body to prevent potential damage to electrical components.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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