P2138 Code on a Buick: Accelerator Pedal Position Sensor 1-2 Correlation (Buick Guide)

P2138 on a Buick is GM's "Accelerator Pedal Position (APP) Sensor 1-2 Correlation." The two sensors inside the gas pedal disagree about where your foot is, so the engine computer cuts power and shows "Engine Power is Reduced." For a 2017–2018 LaCrosse or 2018 Regal there is an open GM special coverage on exactly this — 10 years or 150,000 miles, so many of those Buicks are still inside it. And for every 2018-and-earlier Buick, GM's fleet-wide bulletin on P2138 starts somewhere other than the pedal: water in a wiring connector behind the dash.

What Does P2138 Mean?

The accelerator pedal on every Buick in this guide is a sensor, not a cable. The engine computer reads it to decide how far to open the throttle.

Because a false pedal reading could open the throttle when nobody asked, the pedal assembly contains two separate position sensors, each with its own circuits back to the computer. They are scaled differently, but both rise together as the pedal moves, and the computer constantly checks that they agree.

GM's bulletin describes the setting condition: P2138 sets when there is a voltage difference between APP sensor 1 and APP sensor 2 that exceeds a predetermined value for more than a calibrated period of time. The computer then limits throttle response — reduced engine power — and turns on the check engine light.

Notice what that does not say. It does not say the pedal is bad. Anything that distorts one sensor's signal on its way to the computer — a corroded connector, a damaged wire, a tapped-in accessory — produces the same disagreement.

The LaCrosse and Regal Special Coverage

GM Special Coverage N182188250 (August 2019), "Loss of Continuity within Accelerator Pedal Position Sensor Circuit," covers:

• 2017–2018 Buick LaCrosse
• 2018 Buick Regal (the Sportback and TourX generation)
• 2016–2018 Chevrolet Malibu

GM's condition statement: a condition may affect "the electrical connection to one of the vehicle's accelerator pedal position sensors." If it is interrupted, the pedal still works, but the vehicle enters reduced engine power, the check engine light comes on, the "Engine Power is Reduced" message displays, and P2138 is set. GM put the number of U.S. vehicles involved at approximately 586,625.

The terms:

• Coverage runs 10 years or 150,000 miles (240,000 km) from the original in-service date, whichever comes first, regardless of ownership.
• The repair is a new accelerator pedal and sensor assembly as necessary — GM lists 84478055 and 84478060 and tells dealers to choose by VIN using the parts catalog.
• The dealer follows the service diagnostics for P2122, P2123, P2127, P2128 or P2138 first. If diagnosis does not lead to the pedal, further repairs are not covered by this program.
• Owners who already paid for the repair can apply for reimbursement under the program's procedure.

The coverage also includes an engine harness check at the heater hose clamp, but GM limits that step to the Malibu with the 1.5L engine. It does not apply to the Buicks.

For a 2017–2018 LaCrosse or 2018 Regal, ten years from the in-service date runs into 2027–2028, so many are still covered if they are under 150,000 miles. Ask a Buick dealer to check your VIN before you pay for a pedal.

GM's Fleet-Wide Bulletin: Water in the Connector

GM bulletin 07-06-04-019F (November 2017), "Intermittent MIL Illuminated, DTC P2138 with Reduced Engine Power (Repair Instrument Panel (IP) to Body Harness Connector)," applies to 2018 and prior GM passenger cars and light-duty trucks. The NHTSA index lists it against a long run of Buicks, from the 2005 Century, LeSabre and Park Avenue through the LaCrosse, Lucerne, Enclave, Rendezvous, Terraza, Rainier, Regal, Verano, Encore, Envision and Cascada up to 2018.

The pedal signals reach the computer through the instrument panel-to-body harness connector, which GM says may be in and around the left-hand kick panel or inside the instrument panel. GM's cause is water intrusion into that connector. Its examples of leak sources: A-pillar seals, sunroof drain lines, and windshield or cowl sealing.

GM's steps, in order:

1. Verify that no aftermarket equipment is connected to the APP signal or low reference circuits, or to any other ECM/PCM 5-volt or low reference circuit. GM states directly that aftermarket equipment can generate P2138.
2. Run the vehicle system check. If any 5-volt reference codes are set, diagnose those first.
3. Locate the connector, find the water leak (with a hose if needed) and repair it.
4. Inspect the connector terminals for corrosion and debris and repair them.

No parts are listed. The labor operations GM gives are for resealing body joints, the plenum and the windshield. If your Buick's P2138 shows up after rain or a car wash, look for water staining at the driver's kick panel before anything else — and on a Buick with a sunroof, check the drains.

Older Buicks: 2009 LaCrosse and Earlier GM Notes

Special Coverage 14636 (April 2015) covered the 2009 Buick LaCrosse, the 2009 Allure in Canada and the 2009–2011 Chevrolet Impala. GM's cause: a broken solder connection inside the accelerator pedal position sensor, giving reduced engine power and loss of traction control assist, with "Engine Power is Reduced" and possibly "Service Traction Control" and "Service StabiliTrak" messages. The repair was pedal assembly 22742315, and the dealer was directed to diagnose using the relevant codes, P2138 among them. Coverage was 10 years or 120,000 miles, which has now passed for practical purposes; a dealer VIN check is still free.

14636 also points technicians to bulletin 08-06-04-041, "Check Engine Light On, Hesitation, Reduced Engine Power Message, DTCs P1125, P2101, P2120, P2125 or P2138 Set." The NHTSA index lists its B revision against the 2007–2009 LaCrosse. We could read only the index summary, not the GM document.

The index also lists two older GM notes worth knowing, both seen only as summaries:

• 2004–2005 Rendezvous — 05-06-04-032: reduced engine power and SES light with one or more of P0121, P0221, P2119, P2122, P2138, P2176.
• 2005 Rainier — 3624: "SES light due to a P2138 — possible fuel level sensor circuit short." In other words, on that Rainier a fault in a different circuit was documented as a P2138 cause, which is exactly why GM says to check reference circuits before condemning the pedal.

P2138 Is the Pedal, Not the Throttle

GM uses the same two-sensor safeguard in the throttle body, which sets a different code:

• P2138 — the two sensors in the accelerator pedal disagree.
• P2135 — the two sensors in the throttle body disagree. See our P2135 Buick guide.

The best-known Buick throttle program, Special Coverage 14582 for the 2008–2012 Enclave and LaCrosse, 2009–2011 Lucerne and 2010–2012 Regal, used P2135, P0121 and P0221 as its entry codes. P2138 was not one of them. A throttle body sold for a P2138 is the wrong part.

Reduced Power and Why It Matters

Reduced power is the right response when the computer cannot trust the pedal; acting on a wrong reading could mean unintended acceleration. But it comes on without warning, and limited acceleration in a merge is a real hazard. On the 2009 LaCrosse, GM documented that the fault also took away traction control assist.

Restarting may restore power for a while. That is not a repair. Get the Buick somewhere safe and get it diagnosed.

Symptoms of P2138

• "Engine Power is Reduced" message and limited acceleration
• Check engine light on
• Intermittent loss of throttle response, sometimes after rain or a wash
• "Service Traction Control" or "Service StabiliTrak" messages on some Buicks
• Normal power after a restart, then the fault returning
• Companion codes P2122/P2123 or P2127/P2128 (individual pedal sensor circuits)

Causes and How to Confirm Each

How Is P2138 Diagnosed?

1. Confirm it is P2138, not P2135. Pedal versus throttle decides the part.
2. Check the VIN for N182188250 on a 2017–2018 LaCrosse or 2018 Regal before paying for anything.
3. Record every code, and diagnose any 5-volt reference codes first.
4. Look for aftermarket equipment tied into pedal or reference circuits.
5. Inspect the left kick panel and IP-to-body connector for water and corrosion; find the leak source.
6. Watch APP sensor 1 and 2 on a scan tool through slow and fast pedal sweeps, wiggling the harness.
7. Test the circuits per the wiring diagram.
8. Replace the pedal assembly only when the fault follows the pedal. Order it by VIN.
9. Clear the code and road test, including after rain or a wash if it was intermittent.

Parts and Cost

GM Documents That Name P2138 on a Buick

Have a dealer check your VIN against current coverages rather than relying on any article, including this one.

What Other P2138 Pages Get Wrong

• Paying for a pedal on a 2017–2018 LaCrosse or 2018 Regal. N182188250 covers it for 10 years or 150,000 miles.
• Replacing the throttle body. P2138 is the pedal; Buick's throttle coverage used P2135.
• Going straight to the pedal. GM's fleet-wide bulletin starts with water in a connector behind the dash.
• Ignoring aftermarket accessories. GM says they can generate P2138.
• Skipping the 5-volt codes. GM says diagnose those first; a fault elsewhere can mimic a bad pedal.
• "Just clear it." It is a reduced-power code that can return in traffic.

Frequently Asked Questions

Is my 2017–2018 LaCrosse or 2018 Regal pedal covered?

GM Special Coverage N182188250 covers pedal sensor connection faults with P2138 on those Buicks for 10 years or 150,000 miles from the in-service date. Diagnosis has to lead to the pedal. A dealer VIN check confirms eligibility.

I already paid for a new pedal on my LaCrosse. Can I get reimbursed?

The special coverage includes a customer reimbursement procedure for repairs already paid for that fall within its terms. Contact a Buick dealer or Buick customer assistance with your repair receipt.

Can I drive my Buick with P2138?

Only as far as you need to. Reduced power can come on without warning and limit acceleration. Restarting may restore power temporarily, but the fault will return.

Why does my P2138 come and go with the weather?

GM's 07-06-04-019F describes water entering the instrument panel-to-body harness connector from A-pillar, sunroof drain or windshield leaks. A fault that appears after rain or a wash fits that pattern.

Can a remote starter cause P2138?

GM's bulletin says aftermarket equipment connected to the pedal signal or the computer's reference circuits can generate P2138. If something was installed around the time the code started, check its wiring.

Is P2138 the gas pedal or the throttle body?

The gas pedal. P2138 means the two sensors in the accelerator pedal assembly disagree. The throttle body version is P2135.