P0221
- Powertrain
- Generic (SAE)
- Emissions
Get it looked at now
This is not an emissions-only fault. The near-universal failsafe response is reduced power or forced idle, and it can engage without warning while you are merging or overtaking. The engine is not being damaged, but the loss of throttle authority is a real hazard.
- Safe to drive
- Short local trips only. Reduced power can engage without warning, so avoid highway merging and heavy traffic until it is repaired.
Throttle/Pedal Position Sensor/Switch "B" Range/Performance
Your throttle or pedal position sensor sends two separate signals so the computer can cross-check them. The second signal ("B") is no longer telling the same story as the first, and most vehicles respond by cutting you back to reduced power.
Drive-by-wire pedal and throttle sensors carry two (sometimes three) independent channels, deliberately given different slopes or offsets so the module can verify one against the other. P0221 sets when the B channel is inside its electrical limits — so no circuit-high or circuit-low code — but its value no longer tracks channel A, or no longer agrees with measured airflow and actual throttle plate position. Whether "B" refers to the throttle body sensor or the accelerator pedal sensor depends on the manufacturer.
How the car detected itThe module samples both channels continuously and converts each to a percentage through its own transfer function. If the two disagree by more than a calibrated amount for a calibrated time — or if channel B stalls, jumps, or drops out while A sweeps smoothly — the fault sets. Many strategies also compare commanded throttle angle against measured MAP and airflow, so a throttle plate that cannot reach its commanded position can trip this with no sensor fault at all. The tolerance and timer are vehicle-specific.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Short local trips only. Reduced power can engage without warning, so avoid highway merging and heavy traffic until it is repaired.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- If ignored
- The failsafe recurs and usually becomes more frequent as the sensor or connection degrades. Eventually the vehicle may refuse throttle entirely and idle only. It will not destroy the engine, but it will strand you, and on a vehicle with a sticking throttle plate the underlying carbon problem keeps growing.
- Check engine light
- Solid
- Reduced throttle response, possible limp mode
- Hesitation or surging
- Idle too high or too low
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Resistive track worn | Very common | — |
| 2 | Throttle plate sticking | Common | — |
| 3 | Sensor element aged beyond calibration window | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Freeze frame — engine speed, load, and throttle position at the moment it set
A fault that sets only at wide-open throttle, only near closed throttle, or only at a specific plate angle each points at a different part of the sensor track or a different mechanical bind.
Every other stored code, especially other throttle and pedal codes
P0221 alone means a plausibility problem. P0221 alongside circuit-high or circuit-low codes means a hard electrical fault, and you chase that first.
The connectors at the throttle body and at the pedal assembly
Backed-out terminals, green corrosion, and oil wicking up the harness all produce intermittent signal dropout that reads exactly like a failing sensor. The throttle body connector lives in a hot, vibrating place.
Carbon buildup in the throttle bore and around the plate
A plate that binds or cannot seat where the module commands produces a genuine correlation fault while both sensors are working perfectly.
Aftermarket pedal boxes, throttle controllers, or tuning devices spliced into the harness
These sit directly in the signal path and are a common, easily missed cause of correlation faults.
Record freeze frame and all codes before clearing anything
Tools Scan tool capable of reading freeze frame
Note whether the code is current or historic, how many times it has set, and the operating conditions captured. A correlation code with no accompanying circuit code tells you to look at signal quality and mechanics rather than hunting for a hard short.
ExpectedA range/performance code standing alone, with no simultaneous circuit-high or circuit-low code for the same sensor
Graph both channels while sweeping the pedal slowly, key on and engine off
Tools Scan tool with live graphing, or a lab scope
Sweep the pedal through full travel and back while graphing A and B. First establish which assembly the B channel belongs to on this vehicle. Pedal channels track the pedal with the key on and the engine off. Throttle body channels often do NOT — many vehicles hold the plate at its rest position with the engine off, so a flat throttle-side trace here is expected, not a fault. To sweep a throttle body sensor, use the scan tool's throttle actuator command if the vehicle supports one, or repeat the graph with the engine running. You are hunting for a flat spot, a spike, or a momentary dropout on B that A does not share.
ExpectedOn any channel that is actually being commanded to move, both traces rise and fall smoothly with no steps or gaps. The two will not read the same number — different slopes and offsets are intentional. A throttle-side channel that does not move key-on/engine-off is normal on many vehicles, so confirm plate movement is actually being commanded before calling it a dropout. Actual voltage ranges are vehicle-specific, so compare shape, not absolute value.
Wiggle-test the harness and connectors while still graphing
Tools Scan tool with graphing, heat gun, freeze spray
Flex the harness at the pedal, along its run, and at the throttle body while someone holds a steady pedal position. Include a heat and cool cycle if the fault is known to be temperature-related.
ExpectedA rock-steady trace. Any jump or dropout that follows your hand localizes the fault to that section.
Measure the 5V reference and sensor ground at the sensor connector, circuit loaded
Tools Digital multimeter with backprobes
Backprobe with the sensor connected and running. Measure reference to ground, then do a voltage drop from the sensor ground pin to battery negative.
ExpectedA steady reference near its nominal value, and only a small ground offset — a few tenths of a volt at most. Nominal reference values are vehicle-specific but a 5V reference should sit close to 5.0V.
Check whether another sensor is loading the shared reference circuit
Tools Scan tool, digital multimeter
If the reference reads low, unplug suspect sensors one at a time and watch it recover. One internally shorted sensor elsewhere on the same reference can set correlation codes on the throttle system.
ExpectedReference stays at nominal with everything connected. If it climbs back to normal when a particular sensor is unplugged, that sensor is the fault.
Inspect the throttle bore and plate, clean if carboned, then perform any required relearn
Tools Throttle body cleaner, lint-free cloth, scan tool for relearn
Look for a carbon ring at the plate edge and for a plate that does not move freely by hand with the module unpowered. Do not force an electronic throttle plate open.
ExpectedA plate that moves freely and seats fully. Many drive-by-wire vehicles need a throttle or pedal relearn after cleaning or part replacement — some complete it automatically over a key cycle and a short idle period, others require a scan tool procedure. Check service information for the specific vehicle before assuming either.
Compare commanded versus actual throttle position under load on a road test
Tools Scan tool with datalogging
Datalog commanded throttle angle, actual throttle angle, both pedal channels and MAP through several accelerations, including a full-throttle pull if it is safe.
ExpectedActual position tracks commanded closely and without lag. Actual lagging or hunting behind commanded points at mechanical bind or a failing throttle actuator rather than the sensor element.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Remove an aftermarket throttle controller, pedal box, or tuning device from the signal circuit and return the harness to stock | The correlation fault disappearing once the device is removed and the original connectors restored, with both channels tracking normally through a full sweep | $0–$150 | DIY |
| Repair or replace the affected section of the throttle or pedal harness, or its connector terminals | Signal dropout, or a reference/ground voltage change, that appears when you flex a specific section of harness | $100–$400 | DIY |
| Clean the throttle bore and plate, then perform the manufacturer's relearn | Visible carbon at the plate edge, a plate that binds, or actual throttle position lagging commanded position on a datalog | $100–$250 | DIY |
| Replace the accelerator pedal position sensor assembly | A pedal channel dropping out or failing to track its partner during a slow sweep, with reference and ground verified good at the connector | $150–$450 | DIY |
| Replace the throttle position sensor or the complete electronic throttle body | A throttle-body channel reading implausibly or dropping out with wiring, reference and ground all proven good, or an actuator that cannot hold commanded position | $250–$900 | DIY |
| Repair the circuit or component dragging down the shared 5V reference | Reference voltage below nominal that recovers when one specific sensor is unplugged | $100–$400 | DIY |
| Reprogram or replace the engine control module | Correct signals measured at the module's own pins while the module reports different values — and only after wiring, grounds and sensors are all proven | $600–$2,000 | Shop |
US independent shop rates, parts plus labor. Dealer pricing runs higher, and vehicles where the throttle body is buried under intake plumbing add labor. Some manufacturers require a scan tool with factory-level capability to complete the throttle or pedal relearn, while others relearn automatically over a key cycle and a short idle — check service information before assuming the job is out of DIY reach.
- Assuming "B" always means the throttle body sensor. On many vehicles B is a channel of the accelerator pedal sensor, and the throttle body is replaced for nothing.
- Condemning the sensor because the two channels read different numbers. They are supposed to. Different slopes are the whole point of the cross-check.
- Reading a flat throttle-side trace key-on/engine-off as a dropout. Many vehicles do not command the plate to follow the pedal with the engine stopped, and a good throttle body gets replaced for it.
- Skipping the relearn after cleaning the throttle body or fitting a part, then chasing a brand-new idle complaint that you created.
- Ignoring the shared 5V reference. A single shorted sensor somewhere else drags the reference and sets plausibility codes on the throttle system.
- Fitting a low-quality generic sensor whose transfer curve is slightly different from the original, so the correlation code returns days later.
- Clearing the code and handing the car back without a road test. Many of these faults only appear at a specific throttle angle or temperature.
- Throttle Body
- Throttle Position Sensor B
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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