Emissions

P0137 — Post-Catalyst O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)

Severity: low Safe to drive

What P0137 means

The engine control unit monitors the rear oxygen sensor (after the catalytic converter, bank 1, sensor 2). Its signal normally swings roughly between 0.1 and 0.9 V and helps the ECU judge catalyst efficiency. P0137 sets when the signal stays too low for too long, or the circuit is shorted to ground or open.

Symptoms

  • Check Engine light is on
  • Often no noticeable symptoms
  • Slightly worse fuel economy
  • Failed emissions test
  • Occasionally an unstable idle
  • May appear alongside catalyst codes like P0420

Possible causes

  1. Failed downstream O2 sensor on bank 1
  2. Damaged wiring or corroded connector at the sensor
  3. Open circuit or short to ground in the signal wire
  4. Sensor heater circuit fault
  5. Exhaust leak near the sensor
  6. Worn or clogged catalytic converter
  7. Lean running condition from an engine fault

How to diagnose

  1. Scan all codes and freeze frame data, noting related codes (P0420, heater codes, fuel trim faults)
  2. Inspect the sensor harness and connector for chafing, heat damage, and corrosion
  3. Look for exhaust leaks at the manifold, flanges, and near the sensor
  4. Watch the rear sensor voltage live with a scan tool: a signal stuck low points to the sensor or its circuit
  5. Check heater power, ground, and heater resistance with a multimeter
  6. Test the signal wire from sensor to ECU for continuity and shorts to ground
  7. Review short and long term fuel trims for a lean condition
  8. If the circuit is good, replace the sensor and recheck readings

Common fixes

  • Replace the bank 1 downstream O2 sensor
  • Repair the wiring or replace a damaged connector
  • Fix the exhaust leak near the sensor
  • Replace the catalytic converter only if it is confirmed failing
  • Correct the cause of any lean condition found

Questions and answers

What does P0137 mean?

It means the bank 1 downstream oxygen sensor circuit is reporting low voltage. The ECU sees the post-catalyst signal staying too low and flags a fault in the sensor or its wiring.

Can I drive with code P0137?

Usually yes, as it is not a safety issue. Don't put off diagnosis too long, though, to avoid wasted fuel and an emissions test failure.

Do I need a new catalytic converter for P0137?

Usually not. The sensor, wiring, or an exhaust leak is far more likely. Suspect the catalyst only if the sensor and circuit test good and there are signs of converter wear.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.