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Home / DTC / P2121 — APS(main) range

P2121 — APS(main) range

Detailed page for trouble code P2121.

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Code

P2121

MITSUBISHI P — Powertrain

APS(main) range

Brand: MITSUBISHI
Type: P — Powertrain
AI status
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Page language: EN

Causes

  • Faulty accelerator pedal position (APS) main sensor
  • Damaged, corroded or disconnected wiring/connector to APS
  • Poor reference voltage (5 V) or ground to the sensor
  • Short to battery voltage or short to ground in sensor circuit
  • Internal ECM/TCU input fault
  • Pedal mechanical binding or foreign debris affecting sensor travel

Symptoms

  • Malfunction Indicator Lamp (MIL/Check Engine) illuminated
  • Reduced engine power / limp-home mode
  • Poor or delayed throttle response and reduced acceleration
  • Engine may hold idle but not respond proportionally to pedal input
  • Unusual or inconsistent pedal behavior, possible stalls
  • Diagnostic trouble codes stored for pedal/throttle circuits

What to check

  • Read and record freeze-frame and live data (APS A and APS B voltages, throttle position) with key on and while operating pedal
  • Visually inspect pedal area, sensor and connector for damage, water, or debris
  • Check for related codes (secondary APS, throttle body codes) and recent repairs
  • Inspect wiring harness along routing for pinches, chafing or repair splices
  • Verify proper reference voltage (typically ~5.0 V) and ground at sensor connector
  • Backprobe sensor signal pins; check voltages with key ON engine OFF and during pedal travel

Signal parameters

  • Reference voltage to APS: ~5.0 V (stable with key ON)
  • APS (main) output voltage at rest: typically ~0.5–1.0 V (varies by model)
  • APS (main) output voltage at full pedal: typically ~4.0–4.5 V (varies by model)
  • Secondary APS (if present) should have different offset and correlate linearly with main sensor
  • Expected correlation: both sensors change smoothly with pedal travel; no sudden jumps or drops
  • ECM input resistance: check wiring continuity (should not be open or shorted to ground/B+)

Diagnostic algorithm

  1. Retrieve codes and freeze-frame data; note when fault occurred and vehicle conditions.
  2. Perform a visual inspection of pedal assembly, sensor connector and wiring for damage, corrosion or water entry.
  3. With key ON (engine OFF), backprobe sensor connector: verify 5 V reference, ground continuity and measure signal voltage. Compare to specified ranges and to the secondary sensor if present.
  4. Operate pedal slowly and watch live data for smooth voltage change; look for jumps, dropouts or out-of-range values.
  5. Wiggle test harness and connector while monitoring signal for intermittent faults.
  6. If signal wiring is suspect, disconnect sensor and check for opens/shorts to ground or battery using ohmmeter; repair damaged wiring or connectors.
  7. If wiring and connectors are good but sensor output is out of range or non-linear, replace the APS sensor (follow manufacturer procedures).
  8. After repair, clear codes and road test to confirm proper operation and code does not return.
  9. If fault persists after sensor and wiring verification, verify ECM inputs and grounds, consult vehicle-specific service manual for ECM tests; consider ECM replacement only after all wiring/sensor checks are exhausted.

Likely causes

  • APS (main) sensor failure
  • Open or shorted signal wire between pedal sensor and ECM
  • Loose, corroded or pushed-out connector pins at the pedal or ECM
  • Blown fuse or poor 5 V reference/ground at ECM
  • Damage from previous repairs or routing issues (pinched wiring)
  • Intermittent pedal position due to contamination or worn pedal assembly

Fault status

⚠️ Status
APS(main) range fault — accelerator pedal sensor main circuit out of expected range or inconsistent with secondary sensor. Check pedal sensor, wiring and reference/ground.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-2.5 hours

Similar codes

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406

Browse 406 MITSUBISHI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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