OBD-II Code · Sensors
P0138
O2 Sensor High Voltage (B1S2)
Severity
low
Lowest on our scale. Fix it when convenient.
Can you drive it
Only if running normally
A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety.
Indicative cost (USD)
$150–$400
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
Downstream sensor reading high.
How a shop works this
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.
Note every code that is set, not just this one.
Codes that appear together usually narrow the cause faster than any one of them read alone.
Read the freeze frame — the conditions the fault was recorded under.
Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.
Confirm the fault is current rather than history.
Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.
Then work the likely-causes list in the order shown.
That order is what a shop checks first for this code — wiring and connections before the component they run to, because a connector costs nothing to inspect and is a common answer.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Check engine light
Likely causes
- Failed sensorMost common
- Short to power
- Rich exhaust
Read the full diagnostic procedureP0138 says bank 1 downstream (B1S2) signal is stuck HIGH — over ~0.95V on a narrow-band, indicating a permanently-rich post-cat exhaust.
P0138 says bank 1 downstream (B1S2) signal is stuck HIGH — over ~0.95V on a narrow-band, indicating a permanently-rich post-cat exhaust.
Three possibilities, cheapest-first
- the engine really IS dumping unburnt fuel through the cat (look for upstream rich codes P0172/P0132, a failed coolant-flooded cat, or an exhaust leak upstream of B1S1 that's making the ECU over-fuel); the sensor signal wire is shorted to its own heater B+ wire (especially common when a heat-damaged connector body collapses); or the sensor element itself has failed shorted.
- Backprobe the signal at the ECU key-on engine-off and check for the ~0.45V bias — anything over 1.0V is impossible from a zirconia element and points to a harness short.
- If bias is normal, scan fuel trims — if B1 is running negative trim (subtracting fuel), the rich condition is real and the cat is being damaged; track that root cause before replacing the sensor.
Vehicle-specific patterns2002-2007 Honda Accord / Pilot J30/J35 V6 throws P0138 after a leaking valve-cover gasket lets oil drip onto the downstream sensor over time — fix th…
- 2002-2007Honda Accord / Pilot J30/J35 V6 throws P0138 after a leaking valve-cover gasket lets oil drip onto the downstream sensor over time — fix the VCG first, the sensor often recovers after a few highway pulls.
- 2005-2010Jeep Grand Cherokee 4.7L sets P0138 when the rear heat shield collapses and traps exhaust heat against the sensor pigtail, melting insulation and shorting signal to heater.
- 2008-2013Toyota Highlander / RAV4 2GR-FE throws P0138 from a known-failed batch of Denso downstream sensors (TSB acknowledged).
- 2007-2014Ford Edge / Flex 3.5L Cyclone sets P0138 when the bank 1 (rear) cat starts to crumble internally and dumps unburnt HC at the downstream sensor. Cost band: $150-$350 for sensor; $250-$600 if VCG or upstream rich condition has to be fixed first; $1,000+ if the cat is failing.
Look up another code
P0138 — frequently asked
What does a P0138 code mean?
Downstream sensor reading high.
What causes a P0138 code?
Common causes: Failed sensor; Short to power; Rich exhaust.
What are the symptoms of a P0138 code?
Typical symptoms: Check engine light.
Is it safe to drive with a P0138 code?
Only if running normally. A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety. Estimated repair: $150-$400.
How do I diagnose a P0138 code?
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed. 1. Note every code that is set, not just this one. 2. Read the freeze frame — the conditions the fault was recorded under. 3. Confirm the fault is current rather than history. 4. Then work the likely-causes list in the order shown.
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