P0138 — O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
P0138 sets when the downstream O2 sensor on Bank 1 reports voltage above the calibrated high threshold for a calibrated time — typically indicating a stuck-rich reading or a shorted signal wire. A downstream sensor stuck high often triggers or accompanies P0420. Always check the signal wire for short to power before replacing the sensor.
Fault Overview
Failed downstream O2 sensor, short to power on the signal wire, wiring damage near the exhaust, and rich running conditions affecting the sensor are the primary causes. Disconnect and short-isolation tests prevent replacing a good sensor when the wiring is at fault.
Engine & Platform Coverage
Symptoms
- •P0138
- •downstream O2 high voltage
- •Bank 1 Sensor 2 high
- •P0138 catalyst monitor
- •O2 sensor high voltage
Possible Causes
- 1.Failed downstream O2 sensor
- 2.Short to power on the signal wire
- 3.Wiring damage near the exhaust
- 4.Rich running condition affecting the sensor
Wiring & Circuit Notes
Downstream O2 sensor connector and signal wire. Check for short to power or damaged insulation near the exhaust.
DVOM Specs & Sensor Tests
Sensor voltage (should normally be relatively stable around 0.4–0.6 V post-catalyst). Signal wire voltage with sensor disconnected. Heater circuit integrity.
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 the Downstream O2 Sensor Voltage: Graph the downstream O2 sensor voltage on the scan tool. A healthy post-cat sensor should be relatively stable around 0.4–0.6 V when the catalyst is working.
- 2Step 2 — Disconnect the Sensor and Check Signal Voltage: Disconnect the sensor and check signal wire voltage with the sensor unplugged. Voltage present with sensor disconnected confirms a short to power.
- 3Step 3 — Inspect Wiring for Short to Power: Inspect the signal wire routing near the exhaust for chafing or damage causing a short to power.
- 4Step 4 — Check Heater Circuit: Check the heater circuit resistance and power/ground supply.
- 5Step 5 — Replace Sensor if Circuit Is Good: Replace the downstream O2 sensor if the circuit is proven good and voltage remains high when connected.
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 shorted wiring $50–200; replace downstream O2 sensor $80–280.
Related Codes & Pages
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
Related Systems
Related Fault Codes
Related Sensor Locations
Common Symptoms
Related Articles
AI Diesel Tech — The Technician's Diagnostic Engine
AI Diesel Tech diagnostic logic is built by an ASE Certified Diesel Technician and engineered for professional shop use. Trusted by technicians nationwide — used in shops, fleets, and training programs.
Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.