Often still drivable, but the engine may idle wrong, waste fuel on deceleration, and can stall when you slow to a stop — a real risk in traffic. Fix it soon rather than let a stalling idle strand you. P0510 means the engine control module got an implausible signal from the Closed Throttle Position (CTP) switch — the switch (or switch function) that tells the computer when the throttle plate is fully closed at idle. The ECM either never sees the switch report closed throttle when the engine is idling, or sees it report closed when other inputs say the throttle is open, so it flags the idle-recognition input as faulty.
What P0510 means
P0510 is a Closed Throttle Position Switch circuit/plausibility fault. On the cable-throttle vehicles this code comes from, a discrete switch — often built into the throttle-body or throttle-position-sensor assembly — closes its contacts when the throttle plate returns to its idle (fully closed) stop and opens the moment the driver cracks the throttle open. The ECM uses that clean on/off signal to know it is at idle so it can hand control to the idle-speed strategy, enable deceleration fuel cut-off, and gate features like torque-converter-clutch lockup, EGR flow and EVAP purge. The module cross-checks the switch against engine speed, throttle-position sensor voltage, manifold pressure and vehicle speed. When the switch state disagrees with those inputs — for example it stays "open" during a genuine closed-throttle idle, sticks "closed" while the engine is clearly off idle, or the circuit reads open/shorted — the ECM cannot trust its idle-recognition input and sets P0510. Note: many modern engines use electronic throttle control (drive-by-wire) and have NO separate CTP switch; on those the closed/idle position is derived from the throttle-position or accelerator-pedal sensor, and a fully electronic-throttle vehicle will normally set a TPS/APP code (P0120–P0124, P2135, etc.) instead of P0510. If your vehicle genuinely stores P0510, treat it as a cable-throttle idle-switch concern.
Symptoms
- Check-engine light (MIL) on, sometimes with companion throttle-position or idle-control codes stored alongside it
- Unstable, hunting, or incorrect idle — the engine may idle too high because the ECM never recognizes the closed-throttle state, or surge up and down
- Stalling, most often when slowing to a stop or lifting off the throttle, because the idle-speed strategy is not engaged cleanly
- Reduced fuel economy from disabled deceleration fuel cut-off — the engine keeps injecting fuel on overrun when it should shut the injectors off
- Harsh, early, or missing torque-converter-clutch engagement on automatics that use the closed-throttle signal to schedule lockup
- Slight hesitation or a bump right off idle as the ECM transitions between idle and part-throttle fueling
Common causes
- A worn or failed Closed Throttle Position switch — pitted or sticking contacts that no longer make a clean open/closed transition at the idle stop
- A throttle plate that does not fully return to its closed stop: carbon build-up in the throttle bore, a binding or over-tight throttle cable/linkage, or a sticking return spring holding the plate slightly open so the switch never reads closed
- A misadjusted CTP switch or throttle stop, so the switch trips at the wrong throttle angle (on assemblies where switch position or idle stop is adjustable)
- Open, shorted, or high-resistance wiring in the switch signal or ground circuit between the throttle body and the ECM
- A corroded, loose, or backed-out connector terminal at the throttle-body/switch connector
- A failing throttle-position sensor on integrated units where the closed-throttle signal is derived from TPS voltage rather than a separate contact
- An ECM/PCM internal fault or software issue (rare — confirm the switch, throttle, and wiring first)
Severity & driving advice
Severity: Moderate — Usually a driveability nuisance, but incorrect idle strategy can cause stalling when stopping and hurts economy, so don't leave it indefinitely.
Can I drive? Often still drivable, but the engine may idle wrong, waste fuel on deceleration, and can stall when you slow to a stop — a real risk in traffic. Fix it soon rather than let a stalling idle strand you.
Diagnostic approach
- Scan codes and read freeze-frame data — Record every stored and pending code before clearing anything. Note companion throttle-position, idle-air-control, or idle-speed codes, and read the freeze frame for engine RPM, throttle-position percentage, coolant temperature and vehicle speed at the moment P0510 set — those values tell you whether the fault appeared at a genuine idle or off-idle, which points you toward a stuck-open versus stuck-closed switch.
- Confirm the throttle mechanically returns to fully closed — With the engine off, inspect the throttle bore for carbon and gum, check that the throttle plate snaps back to its idle stop, and verify the cable/linkage is not binding or holding tension. A throttle that rests even slightly open will keep the CTP switch reading "off idle" and can set P0510 with no electrical fault at all. Clean the throttle body and free any binding before condemning the switch.
- Verify the vehicle actually has a discrete CTP switch — Identify whether this is a cable-throttle engine with a separate closed-throttle contact switch or an electronic-throttle (drive-by-wire) engine. If it is electronic throttle, there is no physical CTP switch — the idle position comes from the throttle-position or pedal sensor, and the real concern is a TPS/APP fault or a mismatched sensor calibration, so shift your testing to those circuits.
- Watch the CTP switch state on a scan tool while moving the throttle — Where the switch state is available as a data PID, watch it toggle as you slowly open the throttle from rest. It should read closed/ON at the idle stop and switch to open/OFF just off idle, then return cleanly when released. A state that never changes, changes at the wrong angle, or flickers indicates a bad switch, a maladjusted stop, or a wiring fault.
- Back-probe and measure the switch signal circuit — With the switch connected, back-probe the signal wire and measure it while cycling the throttle by hand. A typical closed-throttle switch pulls the signal between about 0 V and its reference level (often near 5 V or battery voltage) as the contacts open and close — the exact levels depend on whether the circuit is pulled up or grounded. Confirm the voltage snaps cleanly between the two states rather than drifting; a signal stuck at one level points to the switch, its adjustment, or the wiring.
- Test switch continuity, then the wiring to the ECM — Unplug the switch and check continuity across its terminals as you actuate the throttle — it should read near 0 Ω closed and effectively open (infinite) off idle, or the reverse per the design. Then check the signal and ground wires back to the ECM for continuity (below about 1 Ω), and check for a short to ground or to power against a good reference. Wiggle-test the harness and connector while watching for the reading to drop out.
- Adjust or replace the switch, then verify the repair — On adjustable assemblies, set the switch so it just trips at the throttle's closed rest position per the factory procedure; if the contacts are worn or the switch will not toggle cleanly, replace it. After the repair, clear the code, confirm the scan-tool CTP state and TPS voltage transition correctly at idle and off idle, and road-test to make sure P0510 does not return.
FAQ
What is a Closed Throttle Position switch, and how is it different from the throttle position sensor?
The throttle-position sensor reports throttle angle as a continuously varying voltage, while a Closed Throttle Position switch is a simple on/off contact that only tells the computer one thing: is the throttle sitting fully closed at idle, yes or no. On cable-throttle vehicles the ECM uses that clean idle signal to enable its idle-speed strategy and features like deceleration fuel cut-off. P0510 is specifically about that idle-recognition switch, not the analog TPS signal, though on some designs the switch function is derived from the TPS.
Can I keep driving with a P0510?
Usually in the short term, but be careful. Because the ECM cannot cleanly recognize the closed-throttle idle state, the engine may idle too high or hunt, waste fuel by not cutting injectors on deceleration, and — most importantly — stall when you slow to a stop. A stalling idle in traffic is a safety concern, so treat P0510 as something to diagnose promptly rather than ignore.
My car has electronic throttle — why would it set P0510?
Genuine drive-by-wire engines don't have a separate closed-throttle switch; they read the idle position from the throttle-position or accelerator-pedal sensor, and typically set TPS or pedal-sensor codes (P0120–P0124, P2135) instead of P0510. If a fully electronic-throttle vehicle shows P0510, it is a legacy definition being applied to the throttle/pedal signal — focus your testing on the TPS and APP circuits and their calibration rather than hunting for a physical switch that isn't there.
Will cleaning the throttle body fix P0510?
Sometimes, yes. If carbon build-up or a binding linkage keeps the throttle plate from returning fully to its closed stop, the switch never reads "closed" and P0510 sets even though the switch itself is fine. Cleaning the throttle bore and confirming the plate snaps back to its idle stop is one of the first things to check. If the throttle closes properly and the code still returns, the fault is in the switch, its adjustment, or the wiring.
Why does the engine stall with a P0510?
The closed-throttle signal is what tells the ECM to switch into its idle-speed control strategy when you lift off the throttle. If that signal is missing or wrong, the module may not add the extra idle air and fuel it normally would as engine speed drops, so the idle collapses and the engine stalls — most often right as you come to a stop or decelerate. Restoring a clean closed-throttle signal usually cures the stall.