P0138 — Post-Cat O2 Sensor High Voltage (Bank 1 Downstream)

What does P0138 mean?

P0138 is set when the downstream (post-catalyst) oxygen sensor on Bank 1 reports a voltage above the calibrated maximum (typically above 1.0 V) for an extended period. The downstream sensor normally sits at a relatively steady 0.6–0.8 V because the catalyst smooths the rich/lean oscillation it sees coming in. A reading stuck above 1.0 V is either a genuine very-rich condition reaching the catalyst, a shorted signal wire, or a failed sensor biased high.

What are the symptoms of P0138?

• Check Engine Light is illuminated
• Possible mild reduction in fuel economy
• Often no drivability symptoms at all
• Vehicle may fail emissions / smog testing
• Sometimes accompanied by P0172 (rich Bank 1)

What causes P0138?

Repair costs are typical US ranges and vary by make, model, model year, and labor rate. A diagnostic trouble code is a symptom, not a guaranteed failed part — confirm the root cause before replacing anything.

Is it safe to drive with P0138?

In most cases a vehicle stays drivable for short trips with P0138 active, but you should diagnose and repair it promptly. This is a low-severity code — ignoring it can lead to further damage or a failed emissions test. Exact tolerance depends on your specific make and model.

How to diagnose P0138

1. Determine whether the reading is real or sensor-reported

   Read fuel trims at idle and 2500 RPM. If Bank 1 LTFT is around −10 % or worse (the ECM pulling fuel out), the engine is genuinely running rich and the downstream sensor is correctly reporting it. If LTFT is near zero, the engine is fine and the sensor or its wiring is the problem.

   Tools: Scan tool with fuel trim PIDs

2. Inspect for shorted signal wire

   Disconnect the Bank 1 downstream O2 sensor. With the connector unplugged and the key on, scan-tool voltage on the signal wire should fall to bias voltage (0.4–0.5 V). If voltage stays high (above 1 V), the signal wire is shorted to power somewhere between the sensor and the PCM.

   Tools: Scan tool, Multimeter

3. Verify with a known-good sensor

   The downstream sensor is essentially a slow O2 sensor that should not switch rapidly. If a known-good sensor (or the Bank 2 downstream sensor if equipped) reads correctly while the Bank 1 downstream sticks high, the Bank 1 sensor is bad.

   Tools: Scan tool with sensor swap capability

4. Inspect for fuel contamination of the catalyst

   If a separate rich-running fault (P0172, stuck injector) has been present for months, fuel may have entered the catalyst and altered its oxygen-storage behavior — which the downstream sensor sees as a chronic rich signal. This requires the rich cause fixed AND likely catalyst replacement.

   Tools: Diagnostic history review

5. Check for exhaust system damage

   Look for any damage to the exhaust between the catalyst and the downstream sensor that could let oil mist or coolant into the exhaust stream. Coolant entering the exhaust (head gasket, cracked head) creates rich-mimicking sensor readings.

   Tools: Visual inspection, Coolant pressure tester

How do I fix P0138?

• Replace the Bank 1 downstream O2 sensor with an OEM part
• Repair shorted-to-power O2 signal wire
• Address underlying rich condition (P0172) first if present
• Clean or replace corroded O2 sensor connector
• Repair coolant or oil entry into the exhaust (root cause)

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Related diagnostic codes

• P0136 — Post-Cat O2 Sensor Circuit Fault (Bank 1)
• P0141 — Post-Cat O2 Heater Circuit (Bank 1 Downstream)
• P0172 — Fuel System Too Rich (Bank 1)
• P0420 — Catalyst Efficiency Below Threshold
• P0300 — Random or Multiple Cylinder Misfire
• P0171 — Fuel System Too Lean (Bank 1)