P0138 Code on a GMC: O2 Sensor Circuit High Voltage, Bank 1 Sensor 2 (GMC Guide)

P0138 on a GMC is GM's "O2 Sensor Circuit High Voltage, Bank 1 Sensor 2" — the oxygen sensor behind the catalytic converter is reporting more than 1,050 millivolts, a voltage a healthy sensor cannot produce even in a very rich exhaust. So on a Sierra, Yukon, Terrain or Acadia, P0138 is first a wiring and connector question — a signal shorted to voltage — not a "running rich" code. The GMC surprise: for the 2018–2019 Terrain, GM issued software updates tied to P0138 and start/stop hesitation, but GM says plainly that the update does not fix the code — you diagnose and repair P0138 first, then reprogram.

What Does P0138 Mean on a GMC?

Every gasoline GMC rear oxygen sensor we found in GM's tables is a conventional switching sensor. Its signal ranges from about 0.1 V (oxygen present) to just under 1 V (no oxygen). The engine control module (ECM) uses it to judge the catalytic converter and, through post-catalyst fuel trim, to fine-tune the front sensor's fuel control.

P0138 sets when the rear signal is held above GM's ceiling. GM's purpose statement in its truck tables is direct: the DTC "determines if the O2 sensor circuit is shorted to high." The 2008 truck table words it "O2 sensor or circuit is shorted to high," which is a fair summary of where to look — the wiring, the connector, or a sensor that has shorted internally.

Why It Is Not a Rich-Mixture Code

Many pages say P0138 means "the engine is running rich." That does not match GM's test. A rich exhaust makes a good rear sensor read high — around 0.8 to 0.9 V — but not above 1,050 mV. A reading above that means voltage is reaching the signal wire from somewhere it should not: the heater supply inside a damaged harness or connector, another powered circuit, or an internal short in the sensor.

A genuinely rich GMC sets other codes — P0172 for the main fuel trim, or the post-catalyst trim codes. If you have P0138 together with those, fix P0138 first: the post-catalyst trim relies on a trustworthy rear sensor, and GM's tables will not run that test while the rear-sensor faults are active.

What GM's GMC Tables Say

The 2016 Sierra/Yukon table adds conditions worth knowing. The test is for dual-bank exhaust, needs system voltage above 10 V, the heater warm-up complete, more than 5 seconds of run time and an initial delay of more than 235 seconds after the test criteria are met. It is disabled during related faults (throttle, MAF, MAP, injector circuits, excess purge). And on these trucks it only runs with AFM status "All Cylinders active" — so on a 5.3L or 6.2L with Active Fuel Management, a lot of light-load highway time in four-cylinder mode can delay the test.

Which GMCs Set P0138

Any gasoline GMC with a rear oxygen sensor: Sierra and Sierra HD, Yukon and Yukon XL, Savana, Envoy, Canyon, Terrain and Acadia. On single-bank engines P0138 is the one rear sensor; on V6 and V8 GMCs with a sensor behind each converter, bank 2's twin is P0158. GM's 2018–2019 Terrain bulletins label the code "HO2S Circuit High Voltage Sensor 2," without a bank, because those engines have one bank. Duramax diesels and electric GMCs are outside this code.

The Terrain Start/Stop Bulletins (19-NA-062 and 19-NA-077)

GM bulletin 19-NA-062 (April 2019) covers the 2018–2019 GMC Terrain and Chevrolet Equinox with the 1.5L turbo (LYX). Customers may report start/stop performance problems, hesitation from a stop or a check engine light, and technicians may find P0132 (front sensor high) and/or P0138 (rear sensor high).

GM's note is the important part: "This software enhancement will not correct the DTC's set. Diagnose and repair the DTCs first and then update the ECM to the current level of software." In other words, the code is a real fault that needs its own diagnosis; the reprogram addresses the drivability side. GM's unique labor operation is 0.3 hours.

GM followed with 19-NA-077 (November 2019), titled the same way and covering the 2018–2019 Terrain without an engine restriction in the model table. It attributes the hesitation to a "software anomaly" and lists the update's changes: delayed entry into auto-stop, recalibrated restart air and spark for faster torque, a slower generator ramp after restart, and a quicker restart when the brake is released. It repeats that the update "will not correct the P0132 or P0138." GM also warns that reprogramming can erase the intelligent battery sensor's learning, so start/stop may not work until the vehicle has sat shut down for at least four hours.

The Acadia Harness Bulletin (PIT5336)

GM's PIT5336 (October 2014) covers the 2013–2015 GMC Acadia with the 3.6L (RPO LLT). Its code list explicitly includes P0138, with P0131, P0132, P0134, P0137, P0140, the bank 2 sensor codes, the heater codes P0030–P0060 and purge code P0443. Technicians may also find the Emission 1 fuse open.

GM's finding: the engine harness to the bank 1 oxygen sensor chafes on the A/C hose coupling at the back of the engine. A wire rubbed through its insulation is exactly how a signal circuit ends up touching voltage — and how a shared fuse ends up blown. The fix is wiring: repair the damaged circuits, insulate the harness, pull it away from the coupling and tie-strap it to the main harness. No sensor is listed.

The 2016 Acadia and 2017 Acadia Limited power-steering-fluid bulletin (18-NA-009) involves the same bank 1 wiring but lists front-sensor codes and P0137, not P0138.

Savana Cutaway Rear Sensors

GM's PI1006 (June 2013) covers 2011–2013 Savana cutaways with the 4.8L or 6.0L V8 built before November 26, 2012: moisture splashing on hot rear sensors can crack them. Its keyword list includes P0138 and P0158. GM's fix: replace both rear sensors with a crack-resistant design and install a new ECM calibration, because the new sensors are internally wired differently. Emission recall 14096 (2014) then covered certain 2011–2013 gasoline Savana cutaways for the same fracture. These are more than a decade old; most involved vans will have been done, but a dealer VIN check is free.

Where Bank 1 Sensor 2 Is on a GMC

• Sierra, Yukon, Savana and Envoy V8s and the 4.3L V6: bank 1 is the driver side; sensor 2 is behind the driver-side converter.
• Acadia 3.6L and Terrain 3.6L: bank 1 is the rear, firewall-side bank — matching PIT5336's harness location at the back of the engine.
• Terrain 1.5T and 2.0T, Acadia and Canyon four-cylinders, Envoy 4.2L: one bank and one rear sensor, after the converter.

Follow the rear sensor's pigtail from the exhaust to its connector and inspect every clip along the way; confirm positions for your VIN in service information.

Symptoms of P0138

• Check engine light on
• Usually no drivability change — the rear sensor does not run the main mixture
• Start/stop complaints or hesitation from a stop on a 2018–2019 Terrain (19-NA-062/19-NA-077)
• Catalyst and oxygen sensor readiness monitors "not ready"
• Several O2 or purge codes at once if a shared fuse has blown (2013–2015 Acadia)
• Rear sensor voltage pinned above one volt on a scan tool

Causes and How to Confirm Each

How Is P0138 Diagnosed on a GMC?

1. Record every code. P0138 with P0132 on a 2018–2019 Terrain points to 19-NA-062/19-NA-077; with many O2 and purge codes on a 2013–2015 Acadia, to PIT5336.
2. Watch the rear sensor fully warm. Stuck above about 1.05 V is the P0138 picture; a normal rich reading is below it.
3. Unplug the sensor and watch the reading. If it stays high with the sensor disconnected, the fault is in the harness or ECM side.
4. Check the shared fuse. A blown fuse means a short somewhere — that is your lead.
5. Inspect the harness along its full run, especially near the exhaust and, on an Acadia, at the A/C coupling.
6. Inspect the connector for heat damage or moisture bridging the heater and signal pins.
7. Test the signal circuit for a short to voltage.
8. Replace the sensor only when the circuit tests good — by VIN and position.
9. On a 2018–2019 Terrain, update the ECM after the repair, then allow time for the battery sensor to relearn.
10. Clear and drive long enough for the test to run — more than 235 seconds into a drive on the 2016 trucks, with all cylinders active on AFM engines.

Parts and Cost

GM Documents Relevant to P0138 on a GMC

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

What Other P0138 Pages Get Wrong

• "P0138 means the engine is running rich." GM's threshold is above 1,050 mV — beyond what a rich exhaust produces. It is a circuit shorted high.
• "A software update fixes it." For the 2018–2019 Terrain, GM says the update will not correct P0138; repair the fault first.
• "Replace the rear O2 sensor." Unplug it first. If the reading stays high, the sensor is not the problem.
• Chasing P0420 or P2096 first. Catalyst and post-catalyst trim tests depend on a working rear sensor.
• Getting the bank wrong. Driver side on a Sierra or Yukon V8; firewall side on an Acadia 3.6L.

Frequently Asked Questions

Can I drive my GMC with P0138?

Usually yes in the short term — the rear sensor does not control the main mixture. But the catalyst monitor will not complete, and a short that blew a shared fuse may be disabling other emission parts.

Does P0138 mean my GMC is running rich?

Not by GM's definition. The code sets above 1,050 mV, higher than a rich exhaust produces. Look for voltage reaching the signal wire first.

Is P0138 the front or rear sensor?

Rear — Bank 1 Sensor 2, after the catalytic converter. Its low-voltage partner is P0137.

Will a software update fix P0138 on my Terrain?

No. GM's 19-NA-062 and 19-NA-077 say the update addresses start/stop and hesitation but will not correct P0132 or P0138. The code needs normal diagnosis and repair first.

Why did my start/stop stop working after the ECM update?

GM warns that reprogramming can erase the intelligent battery sensor's learning. Start/stop may not work until the vehicle has been shut down for at least four hours.

Why does my 2013–2015 Acadia have P0138 and several other codes?

GM's PIT5336 lists P0138 among codes caused by the bank 1 O2 harness chafing on the A/C hose coupling at the back of the engine, which can also blow the Emission 1 fuse. Repair the harness.

Is there a Buick version of this guide?

Yes — our Buick P0138 guide covers the Enclave side of PIT5336.