P0137 on 2018-2020 Ford F-150 5.0L: O2 Sensor Low Voltage Causes and Fixes

What's Unique About the 2018-2020 Ford F-150

For the 2018-2020 F-150 with the 5.0L V8, the P0137 code is a straightforward diagnostic issue that rarely points to complex system failures. Unlike some platforms where this code can be triggered by PCM software glitches, on this truck it almost always indicates a real hardware fault. While Ford has issued numerous TSBs for PCM software updates causing false catalyst efficiency codes (like P0420) on other engines, there are no widespread TSBs for the 5.0L V8 causing a false P0137. A TSB from 2015 (TSB 15-0073) did list P0137 for the 3.5L EcoBoost, but this is not applicable to the 5.0L in the 2018-2020 range. The most common culprits are the sensor itself failing from age or contamination, or physical damage to the wiring, which is a higher risk on a truck used in harsh conditions. While exhaust leaks are a possible cause, the primary failure point is the sensor or its immediate wiring.

Symptoms You May Notice

• Check Engine Light (Malfunction Indicator Lamp) is illuminated.
• Possible slight decrease in fuel economy.
• Vehicle will automatically fail an emissions/smog test.
• In some cases, a rough or unstable idle may be noticeable.
• Possible hesitation or lack of power during acceleration, though this is less common for a downstream sensor fault.

Most Likely Causes

1. Failed Bank 1, Sensor 2 Oxygen Sensor 🔴 High Probability Oxygen sensors are wear items with a finite lifespan, often degrading after 60,000-100,000 miles. Contamination from exhaust gases, oil, or fuel additives and constant heat cycles cause them to fail and stop producing a correct voltage signal.
   How to confirm: Use an OBD-II scanner with live data to monitor the B1S2 voltage. If the voltage is stuck low (e.g., <0.2V) and does not fluctuate as expected when the engine is revved, the sensor has likely failed. A visual inspection may also show a physically fouled, contaminated, or damaged sensor.
   Typical fix: Replace the Bank 1, Sensor 2 (passenger side, downstream) oxygen sensor.
   Est. part cost: $40-$120
2. Damaged O2 Sensor Wiring or Connector 🟡 Medium Probability As a truck, the F-150's underbody wiring is exposed to road debris, water, and off-road hazards. The harness can get too close to the hot exhaust and melt. Rodents are also known to chew on the soy-based wiring insulation used in modern vehicles. Corrosion in the connector is also a common failure point.
   How to confirm: Visually inspect the entire wiring harness from the O2 sensor to where it connects to the main loom. Check for frayed wires, melted insulation, and corrosion or moisture inside the connector pins. A multimeter can be used to check for continuity and voltage at the connector.
   Typical fix: Repair the damaged section of wire or clean/replace the connector. In some cases, re-pinning the connector is necessary.
   Est. part cost: $5-$50
3. Exhaust Leak ⚪ Low Probability Exhaust manifold gaskets and flange connections can develop leaks over time due to heat cycles and corrosion. A leak before the O2 sensor allows outside oxygen to enter the exhaust stream, which fools the sensor into reading a lean (low voltage) condition.
   How to confirm: Listen for a ticking or hissing sound from the exhaust, especially when the engine is cold. A visual inspection may reveal black soot trails around a gasket or flange, indicating a leak. A shop can perform a smoke test to pinpoint the leak's exact location.
   Typical fix: Replace the leaking gasket or repair the cracked exhaust component.
   Est. part cost: $20-$200

Rare But Worth Checking

• Powertrain Control Module (PCM) Fault: This is extremely rare. The PCM is the last component to suspect. All other possibilities, including the sensor, wiring, and exhaust system, must be definitively ruled out before considering a PCM issue. A user on an F-150 forum was trying to diagnose P0137 and needed to check PCM pinouts, highlighting the complexity of this step.

Diagnosis Steps

1. Connect an OBD-II scanner and confirm that P0137 is the active code. Note any other codes present, especially heater circuit codes (P0141, P0036).
2. Using the scanner's live data function, monitor the voltage for 'O2S12' (Bank 1, Sensor 2). On a fully warmed-up engine at idle, a healthy downstream sensor should show a relatively steady voltage, typically between 0.5V and 0.8V. If the voltage is stuck low (e.g., <0.2V) and doesn't respond when the engine is revved, this confirms the fault condition.
3. Perform a thorough visual inspection. With the engine off and cool, get under the truck on the passenger side. Locate the rear oxygen sensor after the catalytic converter.
4. Inspect the sensor's wiring harness for any signs of melting, chafing, or physical damage. Follow the harness to the electrical connector and check for corrosion, bent pins, or a loose connection. Pay close attention to where the harness might contact the exhaust or frame.
5. Inspect the exhaust system around the sensor for any signs of leaks, such as black soot stains or audible hissing/ticking when the engine is running.
6. If the wiring and exhaust system appear intact, the most likely cause is a failed oxygen sensor.
7. To be certain, you can use a multimeter to test the sensor. Check the heater circuit resistance (if a heater code like P0141 is also present) and check for voltage at the connector harness coming from the PCM. However, for most DIY mechanics, replacing the sensor is the next logical step 🎬 See this easy walkthrough for replacing your O2 sensors. given its high failure rate and affordability.

Parts You'll Likely Need

• Oxygen Sensor (Bank 1, Sensor 2) (OEM #JL3Z-9G444-B) — This is the most common failure point for a P0137 code. The sensor degrades over time and stops sending a correct voltage signal. The OEM part number is well-documented for this application.
  Trusted brands: Motorcraft (OEM), NTK (Often the original equipment manufacturer), Denso, Bosch
  OEM price range: $55-$120
  Aftermarket price range: $40-$80

Related Codes That Often Appear With This One

• P0141 — P0141 indicates a fault in the heater circuit for the same O2 sensor (Bank 1, Sensor 2). These sensors have an internal heater to get them to operating temperature quickly. A failure within the sensor can often trigger both a circuit low voltage code (P0137) and a heater circuit code (P0141). Seeing them together strongly points to a failed sensor or a wiring/fuse issue.
• P0036 — Similar to P0141, P0036 is another code for the O2 sensor heater control circuit for Bank 1, Sensor 2. Forum users on f150forum.com frequently report seeing P0137 and P0036 together, with the fix being a new sensor.
🎬 Watch: How to fix P0036 and P0137 codes together.
• P0430 — While P0137 points to the sensor, it can sometimes appear with P0430 (Catalyst System Efficiency Below Threshold Bank 2). This combination can be confusing, as they point to different banks. It could indicate separate issues or a widespread wiring problem. One user on an F-150 forum reported this combination. It's critical to diagnose each code independently.

Technical Service Bulletins (TSBs) & Recalls

• While no TSB directly addresses P0137 on the 2018-2020 5.0L F-150, related bulletins exist. For example, TSB 22-2260 addresses P0420/P0430 on 2.7L EcoBoost F-150s due to sulfur in fuel, showing a precedent for emissions codes being caused by factors other than component failure. TSB 15-0073 from 2015 also listed P0137, but for the 3.5L EcoBoost, not the 5.0L.

Platform-Specific Known Issues

• Wiring Harness Damage: A known issue on F-150s is the O2 sensor wiring harness being improperly routed from the factory, allowing it to touch hot exhaust components and melt. Another common issue is rodent damage to the soy-based wire insulation. Before replacing a sensor, always inspect the full length of the harness for damage.
• Repeated Sensor Failures: One owner on f150forum.com reported repeated failures of the Bank 2 Sensor 2 (P0056), where the dealer replaced the sensor three times. The issue persisted until the connector was re-pinned, suggesting the underlying problem was in the wiring or connector, not the sensor itself. This highlights the importance of diagnosing the entire circuit, not just swapping the part.

Mechanic-Grade Diagnostic Values

• Downstream O2 Sensor (B1S2) Live Data Voltage — expected: Relatively steady voltage between 0.5V and 0.8V on a warm engine at idle.. Failure: Voltage is stuck below 0.2V, or even 0.1V, and does not respond to changes in engine RPM.
• O2 Sensor Heater Circuit Resistance — expected: Typically 6-20 ohms, but should be verified with vehicle-specific service data.. Failure: An open circuit (infinite resistance) or a reading far outside the specified range confirms a failed heater element within the sensor.
• Wiring Continuity from Sensor Connector to PCM — expected: Less than 1 Ω (ohm) of resistance.. Failure: Resistance above 1 Ω indicates a compromised wire (corrosion, partial break) in the harness.
• Signal Wire Integrity Test (Ford non-bias system) — expected: With the sensor unplugged and key on, probing the signal wire pin on the harness side with a test light connected to battery positive should drive the voltage high on a scan tool (e.g., to 1.0V or higher). The test light should NOT illuminate.. Failure: If the scan tool voltage does not change, there is an open or short to ground in the signal wire between the connector and the PCM. If the test light illuminates, there is a short to ground.

Scan Tool Commands That Help

• Ford IDS (Integrated Diagnostic System): Datalogger - monitoring O2 sensor PIDs (O2S12) — This is the primary function used to view the live voltage data from the Bank 1 Sensor 2 to confirm if it is stuck in a low state, which is the trigger for P0137.
• Ford IDS (Integrated Diagnostic System) or equivalent high-level scanner: KOER (Key On, Engine Running) Self-Test — This test commands the PCM to run a series of system checks, including the O2 sensor circuits, and can help confirm if the fault is currently active.

Wiring & Ground Locations

• Main Battery Ground — On the body structure just in front of the washer fluid reservoir in the engine bay.. A poor main battery ground can cause a multitude of floating voltages and unpredictable electrical issues across the vehicle, potentially affecting sensor readings. While not a direct cause, it's a foundational check for any electrical fault.
• Frame to Body Ground Strap — Underneath the truck, there are multiple ground straps connecting the main frame to the body. One key location is under the passenger side door area.. These grounds are highly susceptible to corrosion and rot, especially in salt-belt regions. A failing body ground can create a poor ground reference for the PCM and sensors, leading to incorrect voltage readings like P0137.
• PCM Connector C1381E — The center connector of the three large PCM connectors located on the firewall.. The 'E' stands for Engine, and this connector carries most of the engine sensor signals, including the O2 sensor signal and ground wires. Testing for continuity or voltage drop requires probing specific pins at this connector.
• Cylinder Head Ground Strap — Attaches from the firewall to the rear of the driver's side cylinder head.. A loose or corroded engine-to-chassis ground can cause issues with sensor signals that use the engine block as a ground path. A forum user with a 2019 5.0L found this strap completely disconnected.

Real Owner Repair Stories

• f150forum.com user RussellRego (2019 Ford F-150 5.0L) — Check Engine Light with multiple permanent DTCs: P0036, P0137 (Bank 1 Sensor 2 codes), and P0430 (Bank 2 catalyst code).
  ❌ Tried (didn't work) Replacing the Bank 1 Sensor 2 O2 sensor with an OEM part did not clear the codes., Clearing codes with a standard scanner did not remove the 'permanent' DTCs., Checking for continuity on the O2 sensor harness wires to the PCM connector showed no breaks.
  ✅ What actually fixed it The user found a loose Black/Yellow wire in the passenger side tail light harness connector. After tightening this seemingly unrelated wire, the P2243 code disappeared, and they were able to proceed with a drive cycle to clear the remaining permanent codes. This highlights how CAN bus issues can sometimes manifest as sensor codes.

OEM Part Supersession History

• JL3Z-9G444-B → No supersession found. This remains the current OEM part number for the Bank 1, Sensor 2 (and Bank 2, Sensor 2) on the 2018-2020 F-150 5.0L. — N/A
  Heads up: The Bank 1 (passenger side) and Bank 2 (driver side) downstream sensors use the same part number, JL3Z-9G444-B. However, be aware that the Bank 1 Sensor 1 (upstream) is a different part number (HC3Z-9G444-B). Ordering the wrong position will lead to a failed repair.

Model Year Variations Within This Range

• 2018-2020: The 2018-2020 5.0L V8 uses conventional 4-wire Zirconia downstream oxygen sensors. This is important because some other Ford engines (like EcoBoost) may use wideband (UEGO) sensors, which have 5 or 6 wires and are tested differently. For P0137 on this specific truck, standard voltage and resistance tests apply.

Diagnostic Flowchart

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0141 — O2 Sensor Heater Circuit Bank 1 Sensor 2. Often appears alongside P0137 if the sensor's internal heater or its wiring circuit has failed.
• P0036 — HO2S Heater Control Circuit Bank 1 Sensor 2. Frequently reported by F-150 owners simultaneously with P0137, indicating a shared wiring harness issue or complete sensor failure.
• P0430 — Catalyst System Efficiency Below Threshold Bank 2. Reported by 5.0L owners alongside P0137, sometimes pointing to broader emissions or exhaust issues.
• P0420 — Catalyst System Efficiency Below Threshold Bank 1. Addressed in related Ford TSBs (like TSB 22-2260) for emissions codes caused by factors like fuel sulfur content.
• P0056 — HO2S Heater Control Circuit Bank 2 Sensor 2. Shares the same failure modes as P0137, such as melted wiring harnesses or connector pin issues on the opposite bank.

Frequently Asked Questions