P0137 Code on a Cadillac: O2 Sensor Circuit Low Voltage, Bank 1 Sensor 2 (Cadillac Guide)

P0137 on a Cadillac is GM's "O2 Sensor Circuit Low Voltage, Bank 1 Sensor 2" — the oxygen sensor behind the catalytic converter is reporting a signal stuck near zero. GM's tables describe the test as a check for a circuit "shorted to low," but a rear sensor also reads low when outside air gets into the exhaust, so leaks belong on the list too. Two Cadillac surprises: on the 2015–2018 Escalade 6.2L, GM's PIP5644 says P0137 can be set by a software anomaly — when it appears with three other O2 codes the fix is to clear codes, not buy sensors — and on the 2.0T ATS, CTS and CT6, GM's PIP5529B ties P0137 to loose intake manifold bolts.

What Does P0137 Mean on a Cadillac?

Every Cadillac rear oxygen sensor we found in GM's tables is a conventional switching sensor, producing roughly 0.1 V when it sees oxygen and close to 0.9 V when it does not. Behind a healthy catalytic converter the signal is calmer than the front sensor's and usually sits toward the middle-to-high side. The ECM uses it to judge the converter (P0420/P0430) and to fine-tune fuel through post-catalyst fuel trim (P2096/P2097).

P0137 sets when that signal sits at the floor while the engine should be producing a normal mixture. GM's 2015 table for the Cadillac 3.6L V6 group (LFX and LF3 in the ATS, CTS, SRX and XTS) defines it this way:

• Signal below 50 millivolts.
• Only in closed loop with the commanded mixture near stoichiometric and moderate load, all injectors on, and no throttle, MAP, EVAP, injector or ethanol-sensor faults active.
• 320 failures out of 400 samples, sampled every 100 ms.
• Type B — normally two failing trips before the light.

GM's description: "This DTC determines if the O2 sensor circuit is shorted to low." GM's 2007 Northstar table adds the other half of the story: it checks for a short to low "by checking for a lean condition during steady throttle." In other words, the ECM cannot tell a grounded wire from a rear sensor that genuinely sees oxygen — your diagnosis has to.

Which Cadillacs Set P0137

On a four-cylinder Cadillac, P0137 is simply the rear sensor. On V6 and V8 models with a sensor behind each converter, the bank 2 equivalent is P0157. Lyriq and Optiq EVs have no oxygen sensors.

The Escalade Software PI (PIP5644)

GM's Preliminary Information PIP5644 (May 2019) covers the 2015–2018 Cadillac Escalade with the 6.2L L86, plus Silverado, Sierra, Suburban, Tahoe and Yukon with the 4.3L, 5.3L and 6.2L. The title says it plainly: "E92 Controller with four O2 sensor DTC's P0132, P0137, P0152, P0157."

GM's condition: check engine light on and the vehicle runs rough, within the first five minutes after starting. GM's cause: "a software anomaly that allows all four DTC's P0132, P0137, P0152, P0157 to set." GM is specific that all four codes must be in history for the PI to apply.

GM's correction at the time: the condition was under investigation. Evaluate the vehicle for other concerns; if none are found, clear the codes and release the vehicle. GM assigned a unique 0.3-hour "check and clear codes" labor operation.

The lesson: on a 2015–2018 Escalade, P0137 together with P0132, P0152 and P0157 is a pattern GM recognizes. P0137 alone, or P0137 that keeps coming back, still needs normal diagnosis.

The 2.0T Intake Bolt Bulletin (PIP5529B)

GM's PIP5529B (September 2019) covers the 2013–2017 ATS (LTG 2.0T and LCV 2.5L), 2014–2017 CTS (LTG) and 2016–2017 CT6 (LTG). Customers may report a check engine light, hesitation or stalling, with any combination of P0068, P0101, P0106, P0131, P0137, P0171, P0324, P0326, P0331 and P0506.

If service information does not lead to a fix, GM says to inspect the intake manifold bolts for being loose. If they are, remove one bolt at a time, add medium-strength threadlocker and torque them in sequence to specification — and do not replace the gasket unless it still leaks after torquing. A lean engine running on unmetered air leaves extra oxygen in the exhaust, which both oxygen sensors report.

Leaks and Water: The Other Low-Reading Causes

Exhaust leaks. GM's service bulletin 20-NA-065 (August 2022) covers 2000–2022 GM gasoline cars and trucks, Cadillac included, with a MIL and fuel trim or oxygen sensor codes — P0137 and P0138 among them. If normal diagnosis does not find the cause and an intake or exhaust leak is suspected, GM recommends its variable pressure leak detector (smoke), an ultrasonic leak tester, or soapy water with low-pressure air fed into the tailpipe. GM notes these methods are "very good for finding small leaks around the O2 sensor embossments."

A small leak at a flange or sensor boss between the converter and the rear sensor can pull in air on exhaust pulses and drag the rear reading low.

Water in the connector. For the 2004 CTS, GM issued bulletin 01585, titled "Post O2 water intrusion," listing P0137, P0138, P0140, P0141, P0157, P0158, P0160, P0161, P2232 and P2235. We saw only the NHTSA summary, so we cannot give GM's repair; the title tells you where to look on any Cadillac with rear O2 codes.

Northstar and Catera Exceptions

• 1998–1999 Eldorado, Seville, DeVille: GM's tables list the rear sensor as Bank 1 Sensor 3 — P0143 (below 49 mV), P0144 and P0146 — and contain no P0137 row. On those cars, a rear-sensor low-voltage fault shows as P0143. The 1997 and 2000–2003 tables do list P0137.
• 2000–2003 Northstar: P0137 has a second trigger — the rear sensor reading lean (below about 575 mV) during power enrichment, when the engine is deliberately running rich.
• Catera (Opel-built 3.0L L81): GM's 1997–1998 tables set P0137 below −0.148 V. The 1999–2001 tables have no P0137 row; a rear sensor reading below 0.04 V sets P0136 "circuit continuity" instead.

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 P0137

• Check engine light on
• Often no driveability change — the rear sensor does little fuel control
• Rough running in the first minutes after start on a 2015–2018 Escalade when all four PIP5644 codes are present
• Hesitation or stalling on a 2.0T with intake bolt looseness (PIP5529B)
• Catalyst monitor not completing; possible P0420 or P2097 alongside
• Failed emissions inspection because of the light or incomplete monitors

Causes and How to Confirm Each

How Is P0137 Diagnosed on a Cadillac?

1. Record every code. On a 2015–2018 Escalade, check whether P0132, P0152 and P0157 are also in history (PIP5644).
2. Check the VIN for PIP5529B on a 2013–2017 ATS, 2014–2017 CTS or 2016–2017 CT6.
3. Read fuel trims and the front sensor. If the whole engine is running lean, fix that first.
4. Watch the rear sensor fully warm in closed loop. Then add fuel briefly — a good rear sensor rises toward 0.8–0.9 V within a few seconds.
5. Leak-test the exhaust from the converter back past the sensor, using smoke or low-pressure air as in 20-NA-065.
6. Inspect the connector for water, corrosion and pushed-out terminals.
7. Unplug and test the circuit for a short to ground and for continuity to the ECM.
8. Replace the sensor only when the circuit, connector and exhaust check out.
9. Clear and verify over two trips.

Parts and Cost

GM Documents Relevant to P0137 on a Cadillac

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

What Other P0137 Pages Get Wrong

• "Replace the rear O2 sensor." GM's Cadillac documents point to software (Escalade), intake bolts (2.0T), leaks and water first.
• "It means the catalytic converter is bad." A bad converter usually makes the rear sensor switch, which sets P0420 — not a signal stuck below 50 mV.
• Ignoring exhaust leaks. GM wrote 20-NA-065 because small leaks around sensor bosses fool oxygen sensors.
• Assuming every Northstar uses P0137. The 1998–1999 tables use Bank 1 Sensor 3 codes for the rear sensor.
• Treating one code like four. PIP5644 applies only when all four O2 codes are in history.

Frequently Asked Questions

Can I drive my Cadillac with P0137?

Usually, yes, for a short time — the rear sensor does little fuel control. But the catalyst monitor cannot complete, so you will not pass an emissions inspection until it is fixed.

Is P0137 the upstream or downstream sensor?

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

My Escalade has P0132, P0137, P0152 and P0157. Do I need four sensors?

On a 2015–2018 Escalade 6.2L, GM's PIP5644 describes that exact set as a software anomaly. If a technician finds no other concern, GM's instruction was to clear the codes. Ask a dealer to check current bulletins for your VIN.

Can an exhaust leak cause P0137?

Yes. A leak ahead of the rear sensor lets oxygen in and drives the reading low. GM's 20-NA-065 recommends smoke or air-and-soap testing.

Why does my ATS 2.0T show P0137 with P0171?

Both point to a lean engine. GM's PIP5529B lists that combination on 2013–2017 ATS and 2014–2017 CTS 2.0T cars and points to loose intake manifold bolts once normal diagnosis is done.

What is the opposite code?

P0138 — the same rear sensor reading too high. The rear heater code is P0036.

What is the bank 2 version?

P0157, on V6 and V8 Cadillacs with a rear sensor on each bank.