OBD-II Code P2138: Accelerator Pedal and Throttle Position Sensor Correlation Error

What Does P2138 Mean?

P2138 indicates your car's Powertrain Control Module (PCM) receives conflicting data between the gas pedal's request and the engine throttle's actual position. Modern "Drive-by-Wire" vehicles use two Accelerator Pedal Position (APP) sensors and two Throttle Position Sensors (TPS). If the voltage signals from these sensors fail to logically agree, the computer triggers P2138 and forces the car into a low-power limp mode for safety.

Technical definition: The official definition is "Throttle/Pedal Position Sensor/Switch 'D'/'E' Voltage Correlation." The PCM detects that voltage signals from two redundant sensors—either within the APP assembly or between the APP and TPS—do not correlate. If the signals differ by more than 0.5V for 2-5 seconds, the PCM flags the input as unreliable and triggers the code.

Can I Drive With P2138?

No — Do Not Drive. Do not drive. Your car enters a reduced-power "limp mode," severely limiting speed to 30-50 mph, which creates a major safety risk in traffic. Continuing to drive causes expensive secondary damage. Limp mode over-fuels the engine, destroying the catalytic converter over time and adding an $800-$2500 repair cost. Pull over and tow the vehicle to a repair shop.

Common Causes

• Faulty Accelerator Pedal Position (APP) Sensor (Very Common) — The sensor assembly attached to the gas pedal wears out over time, sending erratic, drifting, or mismatched voltage signals to the computer.
• Damaged Wiring, Connectors, or Water Intrusion (Very Common) — Wires degrade from heat, vibration, or rodents. On GM vehicles specifically, water leaks into the driver's kick panel connector, shorting the APP circuits (TSB 07-06-04-019F). 🎬 Watch: How to fix the GM kick panel connector issue. Corroded or loose pins mimic a failed sensor.
• Dirty Throttle Body or Faulty TPS (Common) — Heavy carbon buildup makes the throttle plate sticky, forcing the TPS to send incorrect readings. The TPS or internal throttle body motor also fails mechanically over time.
• Outdated PCM Software (Less Common) — The PCM software contains overly sensitive fault thresholds. Manufacturers frequently release Technical Service Bulletins (TSBs) with software updates to fix this false trigger.
• Low System Voltage (Rare) — A failing battery or alternator drops system voltage below 10V, causing unpredictable behavior and signal dropouts from sensitive 5V reference sensors.
• Faulty Powertrain Control Module (PCM) (Rare) — Internal PCM faults, like a failing analog-to-digital converter, trigger correlation errors. Rule out all wiring and software issues before replacing the PCM.

Symptoms

• Reduced Engine Power ("Limp Mode") — The vehicle limits speed to 30-50 mph and accelerates sluggishly to prevent unintended acceleration.
• Check Engine Light is On — The Malfunction Indicator Lamp illuminates immediately, often accompanied by a wrench or throttle warning light.
• Hesitation or No Throttle Response — The engine stumbles, jerks, or ignores gas pedal input entirely, remaining at idle.
• Unstable or Rough Idle — The engine idles erratically, surges, or stalls when stopping because the computer cannot control the throttle plate.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this code.

Common Fixes & Costs

• Replace the Accelerator Pedal Position (APP) Sensor — Parts: $70-$250, Labor: $50-$170, ~0.8 hr book time (DIY)
• Repair Damaged Wiring or Connectors — Parts: $10-$50, Labor: $150-$600, ~2.5 hr book time (Professional)
• Clean the Throttle Body — Parts: $10-$20, Labor: $75-$150, ~1.0 hr book time (DIY)
• Replace the Throttle Body Assembly — Parts: $150-$400, Labor: $100-$215, ~1.2 hr book time (Intermediate)
• Update or Reprogram the PCM/ECM — Parts: $0, Labor: $80-$150, ~0.8 hr book time (Professional)

Used vs. New Parts: Buying Guide

When a used part is worth it: A used Accelerator Pedal Sensor or Throttle Body is a budget-conscious option for older vehicles where new OEM parts are cost-prohibitive.

Donor-vehicle mileage cap: roughly under 80000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• Verify the part number matches exactly.
• Inspect connectors for corrosion or damage.
• Prefer parts from dry climates to minimize moisture damage risk.

Decision logic:

• If Vehicle is under 8 years old or has less than 100K miles → Buy new (OEM or high-quality aftermarket) for reliability and warranty.
• If Vehicle is over 10 years old and budget is the primary concern → A used part from a low-mileage donor is an acceptable risk.
• If The part is known for high failure rates on your model (check TSBs) → Favor a new OEM part, as it contains updated designs to fix original flaws.

Warranty tradeoff: Used parts offer a 30-90 day warranty covering only the part. New aftermarket parts have a 1-year to limited lifetime warranty. New OEM parts carry a 1-year manufacturer warranty.

Worst-case if a used part fails: $200-$500 if a used part fails shortly after installation, representing repeat labor costs.

What Happens If You Wait — Timeline

1. 0-1 month: Intermittent Check Engine Light and 'Reduced Engine Power' message. The car enters limp mode, but the issue resets after restarting the engine. (MPG impact: 0-5%% · Added cost: $0)
2. 1-3 months: Limp mode becomes frequent and harder to clear. The driver notices consistent hesitation. The underlying electrical fault worsens. (MPG impact: 5-10%% · Added cost: $50-$100 in wasted fuel)
3. 3-6 months: The vehicle is persistently in limp mode, making it unsafe to drive. The engine runs rich to prevent stalling, consistently overheating the catalytic converter. (MPG impact: 10-25%% · Added cost: $400-$800 (Risk of catalytic converter damage begins))
4. 6+ months: The catalytic converter melts and clogs from prolonged rich fuel mixtures. The vehicle fails to start and requires a complete exhaust repair. (MPG impact: 25%+% · Added cost: $1,500-$2,500+)

Cost of Not Fixing It

• Immediate: Unpredictable throttle response and 'limp mode' create a major safety hazard. Vehicle stalls or fails to accelerate in traffic. (Added cost: Potential accident costs)
• 1-6 months: Failsafe mode causes a rich fuel condition, overheating and destroying the catalytic converter. (Added cost: $1200-$2800)
• 6+ months: A melted catalytic converter blocks the exhaust, causing severe engine performance issues or a no-start condition. (Added cost: $2000+)

Diagnosis Steps

1. Read Codes and Review Freeze Frame Data
   Use an OBD-II scanner to confirm P2138. Check for related codes (P2135, P0121, P0641) that isolate the fault to the throttle body or 5V reference circuit. Review freeze frame data to see the exact engine conditions when the code set.
   Tools: OBD-II Scanner (Beginner)
2. Check for Technical Service Bulletins (TSBs)
   Search online for TSBs related to P2138 for your specific VIN. Manufacturers release bulletins for known issues, pointing directly to a wiring repair (GM TSB 07-06-04-019F) or a software update (Ford TSB 21-2290).
   Tools: Internet Access (Beginner)
3. Analyze Live Sensor Data
   Use a scanner with live data graphing. Monitor the voltage PIDs for APP Sensor 1 and APP Sensor 2 while slowly pressing the pedal. APP1 sweeps from ~0.5V to ~4.5V; APP2 sweeps at half the rate or opposite. Jumps, dropouts, or a divergence greater than 0.5V indicate a fault.
   Tools: Advanced OBD-II Scanner (Intermediate)
4. Perform a Harness 'Wiggle Test'
   While watching the live data graph, systematically wiggle the wiring harness at the pedal connector, firewall, and PCM. If the voltage drops out during wiggling, you found the exact location of the wiring fault.
   Tools: Advanced OBD-II Scanner (Advanced)
5. Visually Inspect Wiring and Connectors
   Inspect harnesses for frayed wires, melting, or corrosion. Pay extreme attention to the IP-to-body connector in the driver's kick panel on GM vehicles, looking for green copper corrosion from water intrusion.
   Tools: Flashlight (Beginner)
6. Inspect and Clean the Throttle Body
   Remove the air intake hose and inspect the throttle plate for heavy carbon buildup. Clean it carefully with throttle body cleaner and a soft cloth. A sticking plate causes correlation errors.
   Tools: Screwdriver/Ratchet, Throttle Body Cleaner, Rag (Intermediate)
7. Test Sensor Circuits with a Multimeter
   Disconnect the sensor and turn the ignition on. Verify a 5V reference and good ground at the connector pins. Test the internal resistance of the APP sensor's potentiometers; an 'OL' reading indicates an open circuit and a failed sensor.
   Tools: Digital Multimeter, Vehicle-specific Wiring Diagram (Advanced)
8. Perform a Component Relearn Procedure
   After replacing the pedal, throttle body, or disconnecting the battery, use a bidirectional scan tool to initiate the 'Throttle Relearn' or 'Idle Relearn' function. The PCM must learn the new component's baseline values.
   Tools: Bidirectional Scan Tool (Intermediate)

When This Code Triggers (Freeze-Frame Conditions)

• Engine Coolant Temp: 180-200°F (Fully warmed up)
• RPM: 1500-2500 (Cruise / steady throttle)
• Engine Load: 30-60% (Light to moderate acceleration or cruising)
• Vehicle Speed: 35-55 mph (Highway or city cruising speed)

Related Codes

• P2135 — Points specifically to a voltage correlation error between the two sensors inside the throttle body (TPS 'A'/'B'). P2138 is a conflict between the pedal and the throttle body.
• P0121 / P0122 — Indicates a performance problem (P0121) or low input (P0122) from the primary Throttle Position Sensor circuit 'A'. Often appears alongside P2138.
• P0641 — Indicates an open in the 5-volt reference circuit powering the sensors. If P0641 is present, the problem is a wiring short or faulty PCM, not the sensors themselves.

Climate & Environmental Factors

• Humidity / Water Intrusion: Water leaking into key harness connectors (like the GM kick panel connector) causes pin corrosion. This creates voltage discrepancies and triggers P2138.
• Cold Weather: Moisture in connectors freezes and expands, causing intermittent contact. Electrical resistance of aging sensors also falls outside expected ranges at low temperatures.
• Vibration: Constant vibration chafes wires and loosens connector pins ('fretting corrosion'), leading to intermittent signal loss.

How to Talk to a Mechanic About This Code

Say this: "I have a P2138 code causing my car to go into limp mode. I'd like to schedule a diagnostic. Please check for any technical service bulletins and inspect the wiring and connectors, especially if it's a GM vehicle, before recommending a part replacement."

This signals you are an informed customer. It directs the shop to perform a thorough diagnosis instead of replacing the most expensive part, preventing unnecessary parts swapping.

Avoid saying:

• 'Just fix my check engine light.'
• 'My car is acting weird, I think it's the gas pedal.'
• 'Do whatever you think is best.'

Questions to ask before authorizing the repair:

• Did you find the fault by testing the sensor's live data, or was it a visual inspection?
• If you're recommending a part replacement, how did you rule out a wiring or connector problem?
• If you replace the part and the code comes back, is the diagnostic fee applied to further labor?
• Does this repair require a throttle or idle relearn procedure, and is that included in the quote?
• What is the warranty on the part and the labor for this repair?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Recommended if a PCM software update is the known fix (per a TSB) or if the vehicle is under warranty. Otherwise, an independent shop is more cost-effective.
  Best for: Vehicles under warranty., Repairs involving a known Technical Service Bulletin (TSB) for a software update., Complex, intermittent electrical issues requiring specialized tools.
  Downsides: Highest labor rates., Quick to replace entire assemblies instead of repairing a connector or wire. (Typical cost: +50% vs. baseline)
• Independent Shop: Best fit for most P2138 scenarios. An experienced independent technician diagnoses wiring and sensor issues effectively at a lower cost than a dealer.
  Best for: Out-of-warranty vehicles., Diagnosing and repairing common causes like a bad pedal sensor, dirty throttle body, or wiring issues.
  Downsides: Diagnostic skill varies. Ensure the shop has ASE-certified technicians specializing in electrical diagnostics., Lacks tools to perform PCM software updates. (Typical cost: +0% vs. baseline)
• Chain Shop: Acceptable for a straightforward pedal replacement, but AVOID for initial diagnosis. They cannot properly diagnose a wiring fault or perform a PCM update.
  Best for: Simple, bolt-on part replacements if you have already diagnosed the issue yourself.
  Downsides: Technician skill varies dramatically., Lacks advanced diagnostic tools for complex electrical problems., High pressure to upsell maintenance items. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the total estimated repair cost for the P2138 code and any other known issues exceeds 40-50% of your car's private-party value, consider selling or trading it in.

• Car worth $4000, fix is $1800: Borderline. The repair is 45% of the car's value. Get a second opinion before proceeding, as other age-related repairs are imminent.
• Car worth $12000, fix is $700: Fix it. This repair is well under the threshold and is a reasonable cost to maintain a valuable vehicle.
• Car worth $2500, fix is $1500: Walk away. The repair cost is 60% of the car's value. It is not a sound financial decision to invest this much into an older, low-value vehicle.

What Scan Tool You Need for This Code

Minimum: An OBD-II scanner that reads and graphs live data for the accelerator pedal and throttle position sensors.

A basic $20 code reader only shows the P2138 code. It cannot show live voltage signals, which is essential to determine if the fault is the pedal, throttle body, or wiring. Without live data, you are guessing.

Budget: XTOOL A30M, TOPDON TopScan (~$70-150) — Bluetooth dongles that connect to a smartphone. They provide the essential ability to view and graph live sensor data to see if APP and TPS voltages are smooth or erratic.

Mid-range: Foxwell NT710/NT809, XTOOL D7 (~$250-400) — Full-tablet scanners offering bidirectional controls. Critical for P2138 as they command the mandatory 'Throttle Relearn' procedure after replacing parts.

Professional: Autel MaxiCOM MK808/MK906BT, Launch X431 series (~$500-1200) — Provides full bidirectional control, OEM-level diagnostics, advanced data graphing, and the ability to perform software updates or ECU programming.

Rent vs buy: Auto parts stores read codes for free, but their tools lack live data. If a throttle relearn is required, you need a mid-range bidirectional tool, making buying one ($250) a viable alternative to paying a shop $150 for that procedure alone.

How to Clear the Code After You Fix It

1. Reconnect battery if disconnected
2. Use an OBD-II scan tool to clear diagnostic trouble codes
3. Perform a complete OBD-II drive cycle to run all readiness monitors

Drive cycle (~30 minutes): Cold start the vehicle and idle for 3 minutes. Drive in mixed city/highway conditions for 20 minutes, including steady-speed cruises and accelerations. Allow the vehicle to cool down completely.

Readiness monitors affected: Catalyst (CAT) monitor, Oxygen (O2) sensor monitor, Evaporative (EVAP) system monitor

Before emissions retest: drive at least 100 miles to fully set monitors.

Watch out for:

• Disconnecting the battery resets all readiness monitors to 'not ready', causing an automatic emissions test failure.
• The code returns immediately if the underlying electrical fault is not repaired.

Will This Fail Emissions / State Inspection?

Yes — this code typically fails an OBD-II emissions inspection.

• California: An illuminated Check Engine Light is an automatic failure. A full drive cycle must be completed to set all readiness monitors before re-testing.
• New York: The NYS DMV inspection includes an OBD-II scan. An active P2138 code causes an automatic failure.
• Texas: In counties requiring an emissions test, an active Check Engine Light results in an automatic failure.

Most Commonly Affected Vehicles

• Chevrolet / GMC Silverado, Sierra, Malibu, Cruze, Equinox, Tahoe (2007-2018) — Extremely common. TSB #07-06-04-019F points to water intrusion in the IP-to-body harness connector in the driver's kick panel. GM issued Special Coverage N182188250 extending warranties for 2016-18 Malibu/LaCrosse models.
• Nissan Altima, Sentra, Rogue, Qashqai (2013-2019) — Frequently traced to the accelerator pedal assembly. A 'Throttle Body Relearn' procedure is mandatory after disconnecting the battery or replacing parts, otherwise the code persists.
• Ford Focus, Fusion, Escape, Edge (2012-2024) — TSB 21-2290 fixes the code with a PCM software update on 1.5L EcoBoost Escapes. Cheap aftermarket pedal sensors fail frequently; use OEM parts.
• Honda Accord, Civic, CR-V (2013-2017) — Often linked to a dirty or sticking throttle body causing inconsistent TPS signals. Cleaning the throttle body is the primary fix.
• Subaru Legacy, Outback, Forester (2005-2012) — Real-world reports trace the root cause to the accelerator pedal assembly or connector corrosion, despite the code definition pointing to the throttle.
• Volkswagen Jetta, Golf, Passat (2010-2018) — Caused by carbon buildup in the throttle body or water ingress into the gateway control module.

Manufacturer-Specific Notes

• General Motors (Chevrolet, GMC, Buick): TSB 07-06-04-019F details water leaking into the instrument panel-to-body harness connector in the driver's kick panel. The fix involves repairing the connector terminals, not replacing the pedal.
• Nissan: A 'Throttle Relearn' procedure is mandatory after replacing the throttle body or disconnecting the battery. If skipped, P2138 persists even with new parts.
• Ford: Known software issues incorrectly trigger P2138. TSB 21-2290 requires a PCM software update. Ford PCMs are highly sensitive to aftermarket pedal sensors; OEM is required.

Real Owner Stories

2017 Chevy Silverado 1500 with intermittent P2138

The truck randomly entered limp mode with a P2138 code while driving or with just the ignition on.

What they tried:

1. Replaced the accelerator pedal assembly.
2. Replaced the throttle body.
3. Replaced the wiring harness from the pedal to the ECM.

Outcome: Dealerships could not replicate the issue. The owner suspected a bad ECM or an incompatibility with aftermarket sensors. The case highlights the frustration of intermittent wiring faults.

Lesson: For persistent P2138, suspect a wiring issue, a bad ground, or aftermarket sensor incompatibility. OEM parts resolve voltage curve mismatches.

2008 Chevy Silverado with P2138 after parts replacement

The truck had no throttle response and an active P2138 code, even after replacing the throttle body and pedal.

What they tried:

1. Swapped throttle body.
2. Swapped accelerator pedal.

Outcome: A scan tool showed APP sensor voltages were not changing correctly when pressed, proving the problem was still present despite new parts. The fault was in the wiring.

Lesson: Throwing parts at a problem is expensive. Use a scan tool with live data to verify if a new part changed the faulty behavior before moving on.

2008 Buick LaCrosse with P2138 only in the rain

The car entered reduced power mode with P2138 only during or after heavy rain.

What they tried:

1. Replaced the throttle body.
2. Replaced the accelerator pedal.

Outcome: The cause was diagnosed as water intrusion into the IP-to-body harness connector in the driver's kick panel, aligning perfectly with GM TSB 07-06-04-019F.

Lesson: If a fault code appears under specific environmental conditions, suspect an exposed wiring or connector issue before replacing sensors.

How to Prevent This Code From Triggering

• Periodically clean the throttle body. (Every 30,000-60,000 miles.) — Removes carbon buildup that causes the throttle plate to stick. A sticky plate reports a position that doesn't correlate with the pedal, triggering P2138.
• Apply dielectric grease to key connectors. (Whenever connectors are disconnected for service.) — Seals out moisture and prevents corrosion on the pins of the APP sensor and IP-to-body harness connectors, preventing water-intrusion issues.
• Address water leaks into the cabin promptly. (As needed.) — Leaks from windshield seals or sunroof drains drip directly onto critical harness connectors in the kick panel, causing the corrosion that leads to P2138.

Frequently Asked Questions

Is it dangerous to drive with a P2138 code?

Yes, it is extremely dangerous. The code causes a sudden loss of power or completely unresponsive throttle, creating a severe safety hazard in traffic. Tow the vehicle to a repair shop immediately.

I replaced the sensor, but the code came back. What now?

The problem is likely in the wiring or connectors between the sensor and the computer. Perform a 'wiggle test' on the harness while watching live data to locate the short. Ensure you completed the mandatory 'relearn' procedure for your specific vehicle.

What is a 'throttle relearn' procedure?

A throttle relearn recalibrates the vehicle's PCM to the electronic parameters of a new throttle body or pedal sensor. It is mandatory on many vehicles (like Nissan) after service or battery disconnection to clear the code permanently.

What are the most common misdiagnosis mistakes for P2138?

The biggest mistake is replacing the throttle body or pedal without diagnosing the system. The true fault is often a corroded connector (especially on GM vehicles) or a required software update, which are cheaper fixes.

Can a weak battery cause a P2138 code?

Yes. Low system voltage (below 10 volts) causes unpredictable behavior from sensitive electronic modules. It triggers P2138 and forces the PCM to lose its throttle body calibration.

Can I just clear the P2138 code to fix it?

No. Clearing the code turns off the check engine light temporarily, but it returns immediately once the PCM detects the correlation fault again. You must fix the underlying electrical or mechanical problem.

How much does it cost to fix code P2138?

Costs range from $10 for DIY throttle body cleaning to $600+ for complex wiring repairs or throttle body replacement. A professional accelerator pedal replacement averages $150-$350. Always diagnose first, as throwing parts at the problem wastes money.

Key Takeaways

• Do not drive with P2138; the vehicle enters a 30-50 mph 'limp mode' that causes unpredictable throttle response and creates a severe safety hazard.
• Diagnose before replacing parts by checking live scanner data; a faulty $150 accelerator pedal sensor or a corroded wiring connector causes 80% of P2138 codes.
• Search for Technical Service Bulletins (TSBs) first, as GM vehicles frequently require a $200 kick-panel wiring repair (TSB 07-06-04-019F) rather than new sensors.
• Perform a mandatory 'Throttle Relearn' procedure using a bidirectional scan tool after replacing any throttle components or disconnecting the battery, otherwise the code returns immediately.