P0121 The throttle position sensor A signal doesn't agree with what the engine computer expects, even though the voltage is within range

Throttle position sensor A range/performance

P0121 in short

P0121 means the throttle position sensor A reading doesn't match the second sensor or what the computer predicts from airflow. Common causes are a worn sensor inside the throttle body, a sticking or dirty throttle, and loose or corroded wiring. Expect hesitation or limp mode; if the engine loses power or stalls, stop safely.

Most modern engines have two throttle position sensors in the throttle body. Their signals move together as the throttle opens, and on Saab's ME9 system they add up to about 5 volts, so the computer can cross-check them.

P0121 is a range or performance fault, not a simple open or short. Saab's service information sets it when sensor 1 differs from sensor 2 by more than about 9%, or from the throttle angle the computer expects for the measured airflow. A worn sensor track, a throttle that sticks, or a poor connection can all cause that disagreement.

Once it sets, many cars limit throttle opening or engine speed to stay safe. GM bulletins list P0121 with reduced engine power on several models, and owners report sudden loss of acceleration.

Most likely causes

  1. Worn or failing position sensor inside the throttle body

    Common general cause
    Why it ranks here
    On electronic throttles the sensor is built into the throttle body; a Toyota dealer traced P0121 to an internal sensor fault and replaced the throttle body.
    What would change this
    Likelier if the two sensor signals stop mirroring each other during a slow sweep.
    How to test the electronic throttle body

  2. Throttle plate sticking or dirty

    Possible — needs testing
    Why it ranks here
    A throttle that doesn't follow its command makes the position signal disagree with expected airflow; a Ford dealer found a throttle body physically sticking with P0121.
    What would change this
    Likelier if the plate is visibly coked or binds when checked with the ignition off.
    How to test the electronic throttle body

  3. Loose, corroded or damaged sensor wiring or connector

    Common general cause
    Why it ranks here
    Saab's procedure starts by wiggling the harness and checking connectors, and HELLA lists plug contact faults and moisture as common causes.
    What would change this
    Likelier if live readings jump when the harness is moved or the code is intermittent.

  4. Intake air problem such as a wrong air filter or air leak

    Supported by vehicle data
    Why it ranks here
    Because the check compares throttle angle with measured airflow, intake problems can set it; GM told dealers to check for an incorrect air filter and duct on some Cadillac CTS models.
    What would change this
    Likelier on a GM model covered by the bulletin or after air filter work.
    How to test the mass airflow (maf) sensor

  5. Engine computer software or fault

    Possible — needs testing
    Why it ranks here
    TroubleCodes.net rates a computer fault as rare; check for software bulletins before replacing it.
    What would change this
    Only after sensors, throttle and wiring test good.
    How to test the engine control module (ecm / pcm)

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read all codes and freeze-frame data, and note any other throttle or pedal codes and the engine conditions when it set

    Low riskTool: OBD-II scan tool

    • IfOther throttle or pedal codes are stored too

      It meansStart with the shared wiring and the throttle body.

      Go to step 3

    • IfOnly P0121 is stored

      It meansLook at the throttle and the sensor signals.

      Go to step 2

  2. With the ignition off, remove the intake duct and check the throttle plate for heavy deposits or binding, and check the throttle body connector for corrosion or loose pins

    Use cautionTool: Flashlight, basic hand tools

    Warning:Keep the ignition off and fingers out of the throttle bore: on an electronic throttle the motor can snap the plate shut if the key is turned on.

  3. Watch both throttle position signals in live data (or with a meter) while the throttle opens slowly, and wiggle the harness

    Best left to a shopTool: Scan tool, digital multimeter

    Warning:Back-probing sensor and computer circuits can damage terminals or the computer if done wrongly; follow the maker's procedure.

Symptoms

  • Check engine light, sometimes with a reduced-power or throttle warning
  • Hesitation or stumbling when accelerating
  • Unstable idle
  • Sudden loss of acceleration or limp mode Gets more serious: stop and have it checked.
  • Engine stalls while driving Gets more serious: stop and have it checked.

Compare with: Check engine light with loss of power

How to fix

  1. Ask a dealer to check your VIN for bulletins on the throttle body, air filter or engine software; GM and others have published repairs for this code.

  2. Note when it happens (cold start, after rain, at cruise, after air filter work). That helps the shop catch an intermittent sensor or connection fault.

Parts and cost context

  • Electronic throttle body:On electronic throttles the position sensors are part of the throttle body, which is usually replaced as a unit and may need a relearn. Labour varies; get a quote.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Cadillac

  • 2016–2017 · CTS

    Supported by vehicle data

    Engine shut-off after wide-open throttle with P0121; GM points dealers to an incorrect air filter and duct. Whether the bulletin is still current is not verified.

  • PIP5424AStatus unverified

    Engine may shut off after wide-open throttle, P0121/P2101/P2119

    2016–2017cts · issued · NHTSA 10119017

    Engine may shut off after wide-open throttle with P0121, P2101 and P2119; dealers should check that the air filter and duct are correct.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Chevrolet

  • 2006–2018 · Tahoe

    Supported by vehicle data

    GM lists P0121 with idle surge and reduced engine power on high-mileage trucks and cars. Whether the bulletin is still current is not verified.

  • PIP4578BStatus unverified

    Idle surge or reduced engine power with P0121/P2101/P2119 on high-mileage vehicles

    2006–2018tahoe, avalanche, camaro, colorado, impala · issued · NHTSA 10126975

    GM preliminary information for idle surge, reduced engine power mode and DTCs including P0121, P2101 and P2119 on high-mileage vehicles across many GM models.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Toyota

  • 2009 · Corolla Matrix

    Supported by vehicle data

    A dealer traced P0121 and a sudden loss of acceleration to an internal throttle body sensor fault.

  • P0122The throttle position sensor A signal voltage is below its normal range

    Shares causes Common general cause

    P0122 is the same sensor reading too low; a failing sensor or connector can set either code.

  • P0123The throttle position sensor A signal voltage is above its normal range

    Shares causes Common general cause

    P0123 is the same sensor reading too high; check the same sensor and wiring.

  • P2119The electronic throttle body's actual position doesn't match the position the engine computer expects

    Shares causes Supported by vehicle data

    GM bulletins list P0121 together with P2119, a throttle body performance fault, for reduced power.

  • P2101The engine computer sees a problem in the throttle actuator motor circuit or the motor's response, so actual throttle position doesn't match what was commanded

    Shares causes Supported by vehicle data

    GM lists P0121 with P2101, the throttle actuator circuit range fault, on the same complaint.

  • P2135Two throttle or pedal position sensor signals (A and B) that should agree with each other don't

    Same system Common general cause

    P2135 is a direct disagreement between the two throttle position sensors; both point at the throttle body sensors and wiring.

All Electronic throttle and accelerator pedal codes

P0121 questions

Can I keep driving with this code?

Only gently and to a shop. The engine computer may limit power at any moment, so avoid motorways and heavy traffic until it is fixed. If the engine drops to idle, stalls or won't respond to the pedal, pull over safely and have it towed.

Is P0121 the throttle body or the pedal?

Usually the throttle body sensor, its wiring or a sticking throttle. Some definitions also cover pedal sensor A, so a shop should check which sensor the computer flagged before buying parts.

Sources and review

  1. P0121 Throttle Position Sensor A Range/Performance — TroubleCodes.net
  2. Saab 9-3 (2008) ME9 DTC P0121 Throttle Position Sensor 1, Out of Range — Saab Automobile (WIS)
  3. Electronic Throttle Control (Throttle-By-Wire) — AA1Car (Larry Carley)
  4. NHTSA complaint ODI 10449313: 2009 Toyota Corolla Matrix, dealer found P0121 and an internal throttle body sensor fault after loss of acceleration — NHTSA Office of Defects Investigation
  5. NHTSA complaint ODI 10255038: 2005 Ford F-150, dealer found P0121/P2104/P2112 and a throttle body sticking closed after a stall — NHTSA Office of Defects Investigation
  6. P0122 Throttle/Pedal Position Sensor A Circuit Low Input — Kelley Blue Book
  7. Throttle potentiometer (throttle position sensor) — HELLA Tech World
  8. GM PIP5424A: engine may shut off after wide-open throttle, P0121/P2101/P2119 (Cadillac CTS) — General Motors ,
  9. Saab 9-3 (2008) ME9 DTC P0122 Throttle Position Sensor 1, Signal Too Low — Saab Automobile (WIS)
  10. Saab 9-3 (2008) ME9 DTC P0123 Throttle Position Sensor 1, Signal Too High — Saab Automobile (WIS)
  11. GM PIP4578B: idle surge, reduced engine power, P0121/P2101/P2119/P2176 on high-mileage vehicles — General Motors ,
Last reviewed
Written by
Saurabh K.
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