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

P0138 on a Cadillac is GM's "O2 Sensor Circuit High Voltage, Bank 1 Sensor 2" — the oxygen sensor behind the catalytic converter is reporting a voltage that is too high. On every modern Cadillac in GM's tables the limit is 1,050 millivolts, more than a working zirconia sensor normally produces, so GM describes the test as a check for a circuit "shorted to high" — a wiring code, not a rich-engine code. The Cadillac surprise: older Cadillacs played by different rules. The 1997 Northstar set P0138 at just 654 mV, and the 2000–2003 Northstar could set it when the rear sensor read above 200 mV during deceleration fuel cut-off.

What Does P0138 Mean on a Cadillac?

The rear oxygen sensor on a Cadillac is a conventional switching sensor. It produces roughly 0.1 V when the exhaust carries oxygen and up to roughly 0.9 V when it does not. Behind a working converter its signal is steady and usually sits toward the upper-middle of that range. The ECM uses it to judge the converter and to fine-tune fuel.

GM's 2015 table for the Cadillac 3.6L V6 group — the LFX and LF3 in the ATS, CTS, SRX and XTS — defines P0138 like this:

• Signal above 1,050 millivolts.
• 100 failures out of 125 samples, sampled every 100 ms.
• Only after the heater warm-up is complete, the engine has run more than 5 seconds (30 seconds accumulated), all cylinders are active, and an initial delay of more than 150 seconds has passed.
• The commanded mixture must be near stoichiometric and the engine not in power enrichment.
• Type B — normally two failing trips before the light.

GM's description: "This DTC determines if the O2 sensor circuit is shorted to high." Because a healthy sensor rarely reaches that level even when the exhaust is rich, a reading above 1,050 mV points to voltage getting onto the signal circuit — not to a fuel problem.

Which Cadillacs Set P0138

On a four-cylinder Cadillac (ATS, CTS and CT6 2.0T, XT4, CT4), P0138 is simply the rear sensor. On V6 and V8 models with a sensor behind each converter, the bank 2 equivalent is P0158.

Older Cadillacs: Different Thresholds

If you are working on a Northstar-era Cadillac, the generic "P0138 is always a short to voltage" rule does not fully apply:

• 1997 Eldorado, Seville and DeVille: GM's threshold was only 654 mV, checked in closed loop at light throttle with the engine warm. A rear sensor seeing a persistently rich exhaust could reach that, so on these cars a rich condition is a real possibility, not just wiring.
• 1998–1999 Eldorado, Seville and DeVille: GM's tables list the rear sensor as Bank 1 Sensor 3, and the high-voltage code is P0144 (above 952 mV). There is no P0138 row.
• 2000–2003 Northstar: two triggers — above 950 mV, or above 200 mV during deceleration fuel cut-off (DFCO). With the injectors shut off, the exhaust should be nearly all air and the rear sensor should drop low. If it stays above 200 mV, either voltage is getting onto the circuit or fuel is reaching the exhaust when it should not.
• Catera (1997–2001, Opel-built 3.0L L81): GM's Bosch-style tables set P0138 above about 1.08 V (1.5 V in the 2001 table); the 1999 table requires it for 5.1 seconds with the catalytic converter below 800 °C.
• 2004–2006 CTS, STS and SRX 3.6L LY7: GM's tables call P0138 a "short circuit to battery voltage," with thresholds that dropped from 1.5 V in 2004 to 1.05 V in 2006.

The 2004 CTS and SRX Bulletins

The 2004 model year produced three GM documents naming P0138 on Cadillacs. We could only see the NHTSA summaries, so treat the details as corroborated, not verified:

• 040604062 (September 2004), 2004 CTS and SRX: "Check engine light illuminated, diagnostic trouble codes P0138, P0158 set."
• 01366 (November 2003), 2004 SRX: "Service engine soon light on with O2 sensor DTCs P0138, P0151," later expanded to include P0030, P0036, P0050 and P0056 — oxygen sensor heater codes.
• 01585 (2004), 2004 CTS: "Post O2 water intrusion," listing P0137, P0138, P0140, P0141, P0157, P0158, P0160, P0161, P2232 and P2235.

When P0138 and P0158 appear together, both rear sensors are reporting high at once. Two sensors rarely fail on the same day, so look for something they share — water, a connector, a supply or the ECM — before buying a pair.

Leaks, Water and the Rear Sensor

GM's service bulletin 20-NA-065 (August 2022) covers 2000–2022 GM gasoline cars and trucks, Cadillac included, that have a MIL with fuel trim or oxygen sensor codes — P0138 among them. When normal service information does not find the cause and an intake or exhaust leak is suspected, GM recommends a smoke-type leak detector, an ultrasonic tester, or soapy water with low-pressure air fed into the tailpipe, and notes these methods are "very good for finding small leaks around the O2 sensor embossments."

A leak usually drives a rear sensor low, so on its own it is a better fit for P0137. But leaks distort fuel trim and both rear codes appear in GM's list, so a leak test is cheap insurance before you condemn the wiring.

Where Bank 1 Sensor 2 Is on a Cadillac

Sensor 2 is downstream, after the catalytic converter. On the 3.6L V6, bank 1 is the bank with cylinder 1: the right, passenger-side bank on the longitudinal ATS, CTS, STS and CT6, and the rear, firewall-side bank on the transverse SRX (2010–2016), XTS, XT5 and XT6. On an Escalade V8, bank 1 is the driver side. On a Northstar, confirm the bank in service information for your VIN; we have not verified it from a GM document for this article.

Symptoms of P0138

• Check engine light on
• Usually no change in how the car drives
• Companion codes: P0158 (other bank), or heater codes P0036/P0056 on the same sensors
• Catalyst monitor and post-catalyst fuel trim tests not completing
• On older Northstars, possible rich-running signs if the cause is fuel rather than wiring
• Failed emissions inspection because of the light or incomplete monitors

Causes and How to Confirm Each

How Is P0138 Diagnosed on a Cadillac?

1. Record every code and the freeze frame. Note whether P0158 or heater codes are present too.
2. Check your year's definition. A 1997 Northstar's 654 mV limit is a different test from a 2015 V6's 1,050 mV limit.
3. Watch the rear sensor with the engine fully warm. Above about 1.05 V and not moving is the modern P0138 picture.
4. Unplug the sensor. If the scan tool still shows high voltage with the sensor disconnected, the fault is in the wiring or ECM side; if it drops, suspect the sensor or its connector.
5. Inspect the connector for water, corrosion and pushed-out terminals.
6. Test the circuit for a short to voltage and continuity to the ECM.
7. On older Northstars, check for a rich condition — fuel trims, injectors and fuel pressure — before blaming wiring.
8. Leak-test the exhaust if fuel trims look wrong, per GM's 20-NA-065.
9. Replace the sensor only when the circuit tests clean, then clear and verify over two trips.

Parts and Cost

GM Documents Relevant to P0138 on a Cadillac

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

What Other P0138 Pages Get Wrong

• "The engine is running rich." On modern Cadillacs GM's limit is 1,050 mV — a circuit fault. The real rich-mixture code is P0172.
• "It is always a short." Not on a 1997 Northstar (654 mV) or during DFCO on a 2000–2003 Northstar.
• Replacing both rear sensors when P0138 and P0158 set together, instead of looking for the shared cause.
• Skipping the unplug test. It splits wiring from sensor in a minute.
• Assuming every Northstar uses P0138. The 1998–1999 tables report the rear sensor as P0144.

Frequently Asked Questions

Can I drive my Cadillac with P0138?

Usually, for a short time. The rear sensor does little fuel control, but the catalyst monitor will not complete and you will not pass an emissions test until it is fixed.

Is P0138 the upstream or downstream sensor?

Downstream — Bank 1 Sensor 2, after the catalytic converter. The upstream low-voltage code is P0131.

Why do I have P0138 and P0158 together?

Both rear sensors are reading high. GM documented that pair on the 2004 CTS and SRX. Check for a shared cause — water, connectors, wiring or a supply — before replacing two sensors.

Can a bad catalytic converter cause P0138?

Not on a modern Cadillac. A failing converter makes the rear sensor switch more, which sets P0420. It does not push the signal above 1,050 mV.

Does P0138 affect post-catalyst fuel trim?

Yes. A bad rear signal corrupts the trim the ECM learns from it. Fix P0138 before chasing P2096.

Is there a related heater code?

Yes — P0036 for the same sensor's heater. GM's 2004 SRX bulletin grouped heater codes with P0138, so check both.

What is the bank 2 version?

P0158, on V6 and V8 Cadillacs with a rear sensor on each bank.