P0137 Code: O2 Sensor Low Voltage – Fix & Cost

P0137 diagnostic guide — downstream O2 sensor location and MA100 scanner showing live voltage data below 0.21V

STOP — Check the Exhaust Before Spending $400

P0137 Code: O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)

In 55% of cases, P0137 is fixed with a $25 O2 sensor or a $30 wiring repair — not a catalytic converter. This guide shows you exactly how to verify the real fault with live voltage data, saving you hundreds of dollars.

Updated October 2025 13 min read DIY Difficulty: Moderate Fix Cost: $5 – $700

What Does P0137 Actually Mean?

P0137 is set when the Engine Control Module (ECM) sees the downstream O2 sensor on Bank 1 (Sensor 2) reporting a voltage below 0.21V for more than 20 seconds. Sensor 2 sits behind the catalytic converter and should normally produce a relatively flat 0.4–0.7V signal that reflects the converter's oxygen storage — anything persistently lower means too much oxygen is reaching the sensor.

The voltage can drop for three structurally different reasons: a leak introduces ambient air upstream of Sensor 2, the engine is genuinely running lean (excess oxygen exiting the cylinders), or the sensor or its wiring has failed and cannot generate the expected signal.

Bank 1 explained: Inline-4 engines have only one bank and only trigger P0137. V6/V8 engines have two banks — the equivalent code on Bank 2 is P0157, diagnosed identically on the other side of the engine.
Critical: P0137 is a voltage code, not a converter code. P0420 (catalyst efficiency) is what indicates a failing converter. Many drivers replace a perfectly good catalytic converter when an exhaust leak or sensor wiring fault is the actual cause.

Symptoms of P0137

P0137 often has no obvious drivability symptoms — the Check Engine Light is the only warning in many cases. You may, however, notice these secondary effects, especially when the underlying cause is a lean condition rather than a wiring fault:

Check Engine Light — always present; the primary indicator
Fuel economy drop (2–8%) — ECM enriches the mixture as a safety fallback
Rough or unstable idle — typical when a lean condition triggers the code
Strong exhaust odor — present when an exhaust leak is the root cause
Failed emissions test — any pending O2 sensor code prevents passing
Companion codes — P0141, P0171, P0174, or P0420 often appear together
Why mild symptoms? Downstream Sensor 2 is a catalyst monitor, not part of the active fuel control loop (that's Sensor 1's job). The engine continues to run on Sensor 1 data, masking the underlying problem until you notice the Check Engine Light.

What Causes P0137? (Ranked by Frequency)

Check causes in this order — the cheapest and most common first. Most owners resolve P0137 at cause #1 or #2 without ever replacing the catalytic converter.

1

Exhaust Leak Upstream of Sensor 2

A crack in the catalytic converter outlet, a leaking flange gasket, or a loose Sensor 2 bung lets ambient air enter the exhaust stream. The sensor then reads high oxygen content and outputs a permanently low voltage. This is the single most overlooked cause — and the cheapest fix.

Fix: $5–$80 DIY · 30 min
2

Failed Downstream O2 Sensor (Bank 1, Sensor 2)

The sensor's ceramic element degrades over time, or its heater circuit fails, leaving voltage stuck at 0V. Common after 80,000 miles. Confirm with a multimeter resistance test before replacing — at 1,200°F operating temperature, a healthy heater reads about 6–13 ohms.

Fix: $25–$100 DIY · 30 min
3

Corroded Wiring or Damaged Connector

Sensor 2 wiring runs under the vehicle exposed to road salt, water spray, and exhaust heat. Green pin corrosion, melted insulation, or a chafed harness rubbed through by the driveshaft creates resistance that drags voltage below the 0.21V threshold.

Fix: $5–$30 (connector pigtail kit)
4

Engine Running Lean (Vacuum Leak, Weak Fuel Pump)

If the engine runs lean — too much air, not enough fuel — Sensor 2 truthfully reports the excess oxygen. Look for companion codes P0171 (system lean Bank 1) or P0174 (Bank 2). Replacing Sensor 2 alone here changes nothing; fix the lean condition first.

Fix: $50–$400
5

Failed Catalytic Converter (Less Common)

A converter that has lost its substrate cannot store oxygen, so Sensor 2 sees raw, oxygen-rich exhaust. This is rare with P0137 alone — usually P0420 appears first or alongside. Confirmed only after sensor, wiring, and leaks have been ruled out.

Fix: $400–$1,200

What You'll Need

Tools

  • OBD2 scanner (live O2 graphs required) iCarzone MA 100 ›
  • O2 sensor socket (22mm / 7/8") ~$12
  • Ratchet + 3/8" extension
  • Digital multimeter
  • Jack stands + floor jack
  • Penetrating oil (PB Blaster)
  • Safety glasses + gloves

Possible Parts

  • Downstream O2 sensor (Bank 1, Sensor 2) $25–$100
  • O2 sensor connector pigtail $10–$30
  • Exhaust gasket / clamp $10–$40
  • Anti-seize compound $8
  • Aftermarket catalytic converter $150–$400
  • OEM catalytic converter $400–$1,200
Recommended Diagnostic Tool for P0137

iCarzone MA 100 OBD2 Scanner

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Shows live upstream & downstream O2 voltage in millivolts side-by-side — exactly what you need to verify Sensor 2 stays below the 0.21V P0137 threshold. Supports all 10 emissions monitors and works on every 1996+ OBD2 vehicle.

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How to Diagnose P0137 at Home (6 Steps)

Follow these steps in order. The majority of P0137 cases are resolved at Step 3 — finding and fixing an exhaust leak — or Step 5, replacing a failed sensor.

  • 1

    Read All Codes — Not Just P0137

    Plug in your scanner and record every code present. Pay attention to companion codes: P0141 (heater circuit failure), P0171/P0174 (system lean), or P0420 (catalyst efficiency). If lean codes appear alongside P0137, the engine is genuinely running lean — fix the lean root cause before replacing any sensor.

    If P0141 appears alongside P0137, the sensor's heater circuit has failed. The sensor cannot reach operating temperature and reads falsely low. Replace the sensor.
  • 2

    Verify Live O2 Sensor Voltage

    Open live data and watch downstream Sensor 2 voltage at operating temperature (engine warmed up 10+ minutes). A healthy Sensor 2 produces a relatively flat 0.4–0.7V signal. P0137 sets when voltage stays below 0.21V for 20+ seconds. A signal that flatlines near 0V points to an open circuit, exhaust leak, or dead sensor — the waveform chart below shows the difference.

    O2 Sensor Voltage Waveform — Healthy vs. P0137 Fault

    O2 Sensor Waveform Comparison — P0137 Diagnosis Left panel shows healthy system: upstream sensor oscillates 0.1–0.9V, downstream sensor flat at ~0.5V. Right panel shows P0137 fault: downstream sensor flatlined below the 0.21V threshold. 0.9V 0.7V 0.5V 0.21V 0.1V ✓ HEALTHY SYSTEM ✗ P0137 FAULT Sensor 1 (upstream) — always oscillates Sensor 2 healthy — ≈ 0.5V flat Sensor 2 fault — stuck below 0.21V = P0137
  • 3

    Inspect for Exhaust Leaks Before Sensor 2

    With the engine warm and idling, listen and feel along the catalytic converter outlet, exhaust mid-pipe, and the bung where Sensor 2 threads in. A hissing sound, a black soot ring (carbon deposit around a joint), or the feel of pulsing air confirms a leak. Ambient air pulled in upstream of Sensor 2 dilutes exhaust with fresh oxygen — driving voltage permanently low and triggering P0137 without any sensor fault.

    Use a $5 propane torch (unlit) as a smoke source near suspected joints — if RPM changes or you hear it suck through, you have a leak.
  • 4

    Test Wiring and Connector

    Disconnect the Sensor 2 harness. With a multimeter, verify the heater circuit has 12V power and good ground (key on). Test the signal wire for continuity back to the ECM. Look for green corrosion at the pins, melted insulation from exhaust heat, or a chafed connector that has rubbed against the underbody. Even minor wiring resistance can drop the signal below the 0.21V threshold.

  • 5

    Replace the Downstream O2 Sensor

    If wiring and exhaust test clean, the sensor itself has likely failed. Replacement costs $25–$100 and takes about 20 minutes. Apply penetrating oil to the threads 15 minutes before removal — these sensors fuse to the exhaust over time. Use anti-seize on the new sensor's threads (most quality sensors come pre-coated). Clear the code and drive 100+ miles to confirm the repair.

    Use a quality brand (Denso, Bosch, NTK, NGK) — cheap universal sensors often fail within 10,000 miles and re-trigger P0137.
  • 6

    Clear the Code and Complete a Full Drive Cycle

    After any repair, clear all codes and drive at least 100 miles including 10+ minutes of sustained highway speed (55+ mph). This allows the O2 sensor monitor to run and confirm the repair.

    Warning for beginners: Clearing the code does NOT fix the problem. The code will return after one complete drive cycle if the root cause is still present. Only clear codes after performing an actual repair.
    Many state emissions tests require all OBD2 monitors to show "Ready" — not just a cleared code light. Drive the full cycle before heading to the inspection station.

How Much Does P0137 Cost to Fix?

Costs depend entirely on the root cause. Use the diagnosis steps above before committing to any repair. The table below reflects realistic 2025 pricing across independent shops and DIY parts suppliers.

Repair DIY Cost Shop Cost You Save Type
Exhaust leak / gasket repair $5–$40 $120–$300 Up to $260 Try First
Connector or wiring repair $10–$30 $100–$250 Up to $220 DIY Friendly
Downstream O2 sensor $25–$100 $150–$300 Up to $200 DIY Friendly
Vacuum leak repair $15–$80 $150–$400 Up to $320 DIY Moderate
Aftermarket catalytic converter $150–$400 $450–$900 Up to $500 Shop Advised
OEM catalytic converter $400–$1,200 $800–$1,500 Up to $300 Shop Advised
Always inspect the exhaust first: A $5 exhaust gasket or $20 clamp can completely resolve P0137 in many cases — far cheaper than the $25+ sensor most owners replace first. Spend 10 minutes underneath the car before ordering parts online.

Per the EPA's emissions standards ↗ EPA OBD Overview, vehicles with active P0137 codes are legally non-compliant in all states with mandatory emissions testing. The 8-year / 80,000-mile federal emissions warranty may cover your O2 sensor — check with your dealer before paying out of pocket.

Which Cars Get P0137 Most Often?

These models have statistically higher P0137 rates due to sensor placement, harness routing, or documented exhaust issues. Your diagnosis approach may differ by vehicle — see the notes column.

Make Model Years Primary Cause & Notes Risk
Chevrolet Silverado, Tahoe, Impala 2003–2017 5.3L V8 trucks suffer cracked exhaust manifolds that leak ambient air upstream of Sensor 2. Always inspect manifold bolts and the flange gasket first. ↗ Look up your Chevy TSB on NHTSA High
Honda Accord, Civic, CR-V 2007–2018 Downstream O2 sensor wiring chafes against the heat shield over time. Look for melted insulation near the harness clip before condemning the sensor. High
Ford F-150, Explorer, Escape 2005–2018 Exhaust flex pipe cracks just behind the converter introduce ambient oxygen near Sensor 2. Common after 90K miles, especially in salt-belt states. High
Toyota Camry, RAV4, Corolla 2007–2016 Sensor 2 failure around 100K miles is the #1 cause. The downstream sensor is easily accessible and a 20-minute DIY job on inline-4 Toyota engines. Medium
Nissan Altima, Sentra, Pathfinder 2008–2017 Connector corrosion at the Sensor 2 pigtail is common in coastal climates. A $15 pigtail repair kit often resolves the code without sensor replacement. Medium
How to look up your TSB: Visit NHTSA.gov ↗, enter your VIN or year/make/model, and filter by Technical Service Bulletins. Search for "P0137" or "oxygen sensor low voltage." A free TSB reflash from your dealer can resolve the code without parts replacement on certain affected models.

Should You DIY or Call a Mechanic?

DIY If You…
  • ✓ Have an OBD scanner with live O2 sensor graphing
  • ✓ Are replacing an O2 sensor (1–2 hrs, basic tools)
  • ✓ Comfortable working safely under a lifted vehicle
  • ✓ Want to save $100–$250 in shop labor costs
  • ✓ Sensor 2 is accessible (most inline-4 engines)
Use a Mechanic If…
  • → Your vehicle is under the 8-yr / 80K emissions warranty
  • → P0137 paired with P0420 — converter may also be failing
  • → Sensor is seized in the exhaust (broken stud risk)
  • → Multiple lean codes (P0171/P0174) suggest deeper issue
  • → California (CARB) — only CARB-compliant sensors are legal
Never replace the catalytic converter for P0137 alone. P0137 is a voltage code — the converter is rarely the cause. Replacing a $400+ converter without first checking for exhaust leaks, wiring, and sensor failure is the most common (and costly) misdiagnosis in OBD2 repair.

Related Codes You May See With P0137

P0138
O2 sensor high voltage — Bank 1, Sensor 2
P0141
O2 sensor heater circuit malfunction
P0157
O2 sensor low voltage — Bank 2, Sensor 2
P0420
Catalyst efficiency — Bank 1
P0171
System too lean — Bank 1
P0174
System too lean — Bank 2

Frequently Asked Questions

Can I drive with a P0137 code?
Yes, for short distances. P0137 is not an immediate safety hazard, but the ECM can no longer accurately monitor catalytic converter efficiency. If an underlying lean condition is the true cause, prolonged driving can damage the converter and ultimately the engine. Plan to fix it within 1–2 weeks.
Will replacing the O2 sensor fix P0137?
In roughly 50% of cases, yes — but always rule out exhaust leaks and wiring damage first. Replacing a perfectly good sensor that's only reporting a real exhaust leak or lean condition wastes $100 and the code returns within a few miles. Confirm using live voltage data before ordering parts.
What's the difference between P0137 and P0138?
Same sensor (Bank 1, Sensor 2), opposite electrical faults. P0137 means the signal voltage stayed too low (below 0.21V for 20+ seconds), typically from an exhaust leak, lean condition, or open circuit. P0138 means the voltage stayed too high (above ~1.0V), usually from a shorted signal wire, contamination, or a sensor element that has failed shorted.
How do I find an exhaust leak that's causing P0137?
With the engine warmed up at idle, slowly run your hand (carefully — exhaust is hot) along the catalytic converter outlet, mid-pipe, and around the Sensor 2 bung. A leak feels like a pulsing puff of air. Look for a black soot ring around joints — that's carbon deposited by escaping exhaust. A propane torch held unlit near suspected leaks will cause the idle to change if the leak is sucking ambient air in.
Can an exhaust leak cause P0137?
Yes — and it's one of the most overlooked causes. A leak anywhere upstream of Sensor 2 (cracked catalytic converter outlet, leaking flex pipe, loose flange gasket, or even a poorly-seated sensor) lets ambient air enter the exhaust stream. The sensor sees the resulting oxygen-rich gas and outputs a permanently low voltage, triggering P0137 without any actual sensor fault.
Is P0137 the same as P0157?
Same fault, different cylinder banks. P0137 is Bank 1 (contains cylinder #1). P0157 is Bank 2. V6 and V8 engines can trigger both codes simultaneously, requiring separate diagnosis on each bank. Inline-4 engines only have one bank and will only ever trigger P0137.
What scanner do I need to diagnose P0137 properly?
You need a scanner that shows live O2 sensor voltage in millivolts — not just a basic code reader. The critical feature is real-time downstream sensor voltage display so you can confirm Sensor 2 is staying below the 0.21V threshold and verify the fix afterward. The iCarzone MA 100 shows live O2 graphs, all 10 emissions monitors, and freeze-frame data for under $40. A basic $20 reader that only clears codes will not give you the data needed to diagnose P0137 correctly.
Does P0137 mean my catalytic converter is bad?
Not usually. P0137 specifically indicates a voltage problem at the sensor — not a converter efficiency problem (that's P0420). The converter is only a likely suspect when a brand-new sensor still reads low voltage after fixing all exhaust leaks and wiring faults. Replacing the converter as a first step for P0137 alone is one of the most expensive misdiagnoses in OBD2 repair.
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Automotive Diagnostic Specialists

Our team of ASE-certified technicians and OBD-II diagnostic engineers review every article for technical accuracy. Content is based on hands-on diagnostic experience across domestic, Asian, and European vehicle platforms.

10+ years diagnostic experience ASE Certified Last reviewed: October 2025