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DTC codes / P21xx · Fuel and air metering / P218D

P218DThrottle/Fuel Inhibit B Circuit Low

P218D indicates that the powertrain control module (PCM) has detected an abnormally low voltage condition on the throttle/fuel inhibit B circuit, which is a control path used to cut throttle or fuel delivery during certain safety or traction events. The PCM monitors this circuit for expected voltage and flags the code when the signal falls below the calibrated threshold, indicating a short to ground, an open circuit, or a faulty component on that path.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Reduced engine power or limp mode, especially during acceleration or when traction control activates
  • Unexpected throttle response, such as hesitation or a brief cut in power
  • Disabled traction control or stability control systems with their own warning lights
  • Rough running or stalling in rare cases if fuel delivery is intermittently inhibited

Common causes, most likely first

  1. Damaged or chafed wiring in the throttle/fuel inhibit B circuit, often near the throttle body or pedal assembly
  2. Corroded or loose electrical connector at the throttle actuator, pedal position sensor, or PCM
  3. Failed throttle position sensor or accelerator pedal position sensor
  4. Faulty throttle body assembly with an internal short or open in the inhibit circuit
  5. Defective PCM driver or internal circuit for the inhibit B control
  6. Aftermarket electronic throttle controllers or poor previous repairs introducing resistance

How to diagnose and fix it

  1. Inspect the throttle body and accelerator pedal connectors for corrosion, loose pins, or water intrusion and clean or repair as needed
  2. Perform a wiggle test on the harness while monitoring live data for the inhibit circuit voltage to locate intermittent faults
  3. Check the circuit voltage and resistance from the PCM to the throttle body and pedal sensor against manufacturer specifications
  4. Test the throttle position sensor and accelerator pedal position sensor for smooth signal sweep and correct reference voltage
  5. If wiring and sensors test good, substitute a known-good throttle body to rule out an internal fault
  6. Repair or replace damaged wiring sections and clear the code, then perform a throttle relearn or idle relearn procedure
  7. If all external circuits and components pass, diagnose the PCM for a faulty internal driver before replacement

Why this code often misleads

  • A low voltage on the inhibit B circuit is frequently caused by a poor ground connection shared with other engine sensors, not by the throttle body itself.
  • The code can set intermittently during hard cornering or bumps if a harness is chafed, so a static voltage check may pass while the fault is still present.
  • Aftermarket throttle controllers or piggyback tuners that splice into the pedal wiring are a common source of this code on modified vehicles.
  • Some vehicles set P218D when the battery voltage drops during cranking, so always check battery health and charging system before chasing circuit faults.
  • The throttle/fuel inhibit circuit is often tied to the electronic stability control system, so a failing ABS wheel speed sensor can indirectly trigger this code.

Can I keep driving with P218D?

The vehicle may enter limp mode or cut throttle unexpectedly, so avoid aggressive acceleration and high-speed driving until the fault is repaired. It is generally safe to drive gently to a repair shop, but if the engine stalls or power is severely reduced, stop driving and seek professional diagnosis.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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