Error P0140 code is a bank 1 error that indicates a malfunction with the rear oxygen sensor, also known as the post catalytic converter.


The oxygen sensor (O2 sensor) that is located on bank 1, sensor 2 relies on a 0.45V voltage reference generated by the engine control module (ECM). When this voltage is exceeded or not met, the O2 sensor misreads the amount of oxygen in the exhaust. This results in a sensor malfunction or failure. The readouts from this sensor are used in vehicle emission tests; a faulty test can cause the vehicle to fail these tests. While the issue in itself is minor, it may be indicative of a more serious problem within the fueling or exhaust system. It may also lead to more frequent or serious issues involving the engine.

While a sensor failure may sound major, it is in fact a natural part of the vehicle life cycle; sensors will inevitably fail and need to be replaced as the vehicle continues to be operated over time.

Common Symptoms

  • The vehicle’s “Check Engine Soon” or “Engine Malfunction” light will be illuminated
  • While the vehicle’s performance will not be noticeably different, drivers may notice a higher rate of fuel consumption
  • There may be excessive exhaust smoke

Possible Causes

  • The O2 sensor has suffered a shorted heater circuit or signal circuit
  • The P0140 code appears when the vehicle’s engine control module (ECM) detects no activity form the O2 sensor
  • If the O2 sensor does not respond to fuel cutoff or fuel enrichment signals from the ECM, it will be deemed inactive
  • Additionally, the EMC will not detect the O2 sensor if the mass airflow sensor and O2 sensor’s readings are miscalculated or if the ECM itself is malfunctioning
  • Over time, the sensor’s harness connector and nearby wiring may melt due to close proximity or contact with the exhaust system. This has sometimes been known to be the cause

What to Check

Error P0140 can be diagnosed fairly easily. With the engine on and in warm-up mode, use a scan tool and specifically pay attention to voltage data from the bank 1 sensor 2 O2 sensor. If the sensor is working correctly, there will be a gradual back and forth switch above and below 0.45V. If the voltage has become stuck at one value, then there has been a malfunction. The mass airflow sensor data should then be checked for changes when the vehicle’s throttle is engaged or disengaged; ideally it should respond or else there is a sensor problem. The surrounding harnesses and wires should also be checked for signs of melting. Similarly, the ECM should be checked to rule out that it has malfunctioned as well.

How to Fix

If a sensor malfunction has been diagnosed and verified, the best course of action is to replace the faulty O2 sensor at the earliest convenience. The same goes for the ECM if it has proven to be faulty.