Technical Description of OBD-II Trouble Code
Low Voltage in O2 Sensor Circuit (Bank 2 Sensor 2)
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to vehicles equipped with OBD-II. While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
This code indicates a fault in the post-catalyst O2 sensor on Bank 2. To understand this, let’s start with a basic explanation of the catalyst: The Catalytic Converter is used to control tailpipe emissions and “clean up” the exhaust. The Powertrain Control Module (PCM) uses the signal from the heated oxygen sensor behind the catalyst to monitor the efficiency of the catalyst by comparing it to the O2 sensors in front of the catalyst.
If the PCM detects little or no difference in the exhaust quality between the post- and pre-catalyst sensors, it indicates that the catalyst is not functioning properly. In simple terms, the main role of a post-cat sensor is to monitor catalyst efficiency, not control fuel management. Although it can influence the operation of the front O2 sensor, we will focus on the basics for simplicity.
If you have a P0157 code, it does not necessarily mean that your catalyst is faulty (in fact, it usually isn’t). This code essentially indicates that the signal voltage from the Bank 2, Sensor 2 (second O2 sensor at the back of bank 2) is below the normal range for that sensor. This four-wire sensor receives a 0.5 volt reference voltage (usually on the signal wire) and has a ground, as well as a power and ground circuit for the O2 sensor heater element. Changes in oxygen content cause resistance changes in the sensor’s signal circuit, which affects the 0.5 volts on the signal/reference circuit. A properly functioning post-catalyst sensor will exhibit small changes at a slower rate compared to the front (pre-catalyst) O2 sensors. It may switch slightly above and below 0.45 volts at a rate of about 1 switch per second or less (depending on the vehicle). The PCM monitors this changing signal to determine if the catalyst is working properly. If the PCM determines that the post-cat O2 sensor signal voltage remains below a certain threshold for too long, it will trigger the P0157 code.
Symptoms
Rear O2 sensor problems usually do not cause drivability issues since they measure catalyst efficiency (unlike front O2 sensors). However, if you have other codes present, such as lean codes or pre-O2 sensor codes, they may cause drivability problems along with P0143. The following symptoms may be present:
Causes
If you suspect that your engine is running lean, address that issue first, then clear the codes and see if the P0157 code reappears. An engine running lean could trigger this code, but usually there will be lean codes (P0171, P0174) present. The following could be potential causes:
Possible Solutions
It is always recommended to start by checking for air leaks in the exhaust manifold and pipes. Address these issues first.
With the engine running, use a scan tool or voltmeter to check the O2 sensor signal voltage when the engine is warm. You may need to increase the RPM to a fast idle to check the voltage of the bank 2,2 sensor. If the voltage remains low (less than 0.5 volts, which may not be enough to trigger the code, depending on the vehicle) and there are no other O2 sensor codes present in the PCM, it is likely that the sensor is faulty. If the voltage is not consistently low and the O2 sensor appears to be operating fine, it may be intermittently sticking low. In either case, replacing the sensor can be an effective diagnostic step. However, it can also be an expensive diagnostic tool, especially if the issue turns out to be a wiring problem rather than a faulty sensor.
So, if you want to be certain about the problem before spending unnecessarily, follow these steps: 1. Turn off the engine and unplug the connector of the 2,2 O2 sensor. Inspect for corrosion or any other damage. Repair as necessary. Ensure that there is battery voltage and ground present for the O2 sensor heater. If not, diagnose and address that issue first, then retest. The O2 sensor relies on the heater to warm up quickly and start functioning. If the heater element is faulty or not receiving the proper voltage, it may trigger this code. Heater problems usually have associated codes as well. If you find that there is no battery voltage supplying the O2 sensor, use the associated code to diagnose that issue. If you find that the heater element is faulty, replace the sensor.
If the heater element checks out, use a jumper wire to supply a chassis ground to the O2 sensor connector’s ground circuit (PCM side). Now observe the scan tool’s reading for the bank 2,2 O2 sensor. If it is now at approximately half a volt, replace the O2 sensor. If the voltage reading does not change after supplying a ground to the unplugged PCM side of the O2 sensor connector, check the harness for a short circuit to ground. Check for any contact between the harness and exhaust components. Inspect the pins for damage or moisture. Repair as necessary. If no harness problems are found, the PCM may be at fault.
