P0138: OBD-II code P0138 means the downstream oxygen sensor on bank 1 is reading persistently high voltage — either a bad sensor or the catalyst is passing rich exhaust.

Symptoms

• Check engine light is on; engine typically runs and drives normally without obvious performance issues.
• Fuel economy may drop slightly and the exhaust may smell of raw fuel, especially at idle.
• Downstream sensors flagged high often accompany catalyst efficiency codes like P0420 as the cat fails.

Common causes

• A failed downstream O2 sensor with an internally shorted signal element, pinned high.
• A real rich condition passing through a failing catalytic converter — usually paired with P0420 or a misfire code.
• A short-to-voltage on the downstream O2 signal wire between the sensor and the PCM.

What to do?

Read live downstream O2 voltage — it should sit steady around 0.6-0.8 V. If pinned at 0.9 V+ with all other trims normal, replace the sensor. If upstream trims show rich, address the fuel-side cause first. Check for a shorted signal wire before condemning the sensor.