P0137 — O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
P0137 sets when the downstream O2 sensor voltage stays below the calibrated minimum, indicating a lean reading, open circuit, or short to ground. An exhaust leak just before the downstream sensor can pull in oxygen and cause low voltage readings — always inspect for exhaust leaks near the sensor before condemning it.
Fault Overview
Failed sensor, signal wire short to ground or open circuit, exhaust leak near the sensor, wiring damage, and lean running conditions are the primary causes. Correlate with fuel trims and upstream O2 activity to distinguish a true lean condition from a sensor or wiring fault.
Engine & Platform Coverage
Symptoms
- •P0137
- •downstream O2 low voltage
- •Bank 1 Sensor 2 low
- •O2 sensor low voltage
Possible Causes
- 1.Failed downstream O2 sensor
- 2.Signal wire short to ground or open circuit
- 3.Exhaust leak near the sensor
- 4.Wiring damage
- 5.Lean running condition
Wiring & Circuit Notes
Downstream O2 signal and ground circuits. Check for open circuits at the connector and chafed wiring near the exhaust.
DVOM Specs & Sensor Tests
Sensor voltage. Continuity and short-to-ground tests. Heater circuit resistance and operation.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
4 steps locked behind Technician Mode- 1Step 1 — Graph Downstream O2 Voltage: Graph the downstream O2 sensor voltage on the scan tool.
- 2Step 2 — Inspect for Exhaust Leaks Near the Sensor: Inspect for exhaust leaks just upstream of the downstream sensor — a leak draws in oxygen and causes a false lean reading.
- 3Step 3 — Check Wiring for Open or Short to Ground: Check the signal wire for open circuit or short to ground.
- 4Step 4 — Test Heater Circuit: Test the heater circuit resistance and power/ground supply.
- 5Step 5 — Replace Sensor if Circuit and Exhaust Are Good: Replace the downstream O2 sensor if the circuit and exhaust are proven good.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Repair exhaust leak $50–400; repair wiring $50–200; replace downstream O2 sensor $80–350.
Related Codes & Pages
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Summary
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