P0157 O2 Sensor Circuit Low B2S2
OBD-II Trouble Code Technical Description
O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to OBD-II equipped vehicles. Although generic, the specific repair steps may vary depending on make/model.
This code refers to a fault in the post-catalyst o2 sensor on Bank 2. First, a basic run-down of the catalyst: The Catalytic Converter is used to help control tailpipe emissions, to "clean up" the exhaust, if you will. The PCM (Powertrain Control Module) uses the signal from the heated oxygen sensor to the rear of the catalyst to monitor the catalyst's efficiency by comparing it to the o2 sensors in front of the catalyst.
If the PCM sees little or no difference between the exhaust quality in post- and pre- catalyst sensors then it knows that the catalyst is not working properly. To over simplify: a post-cat sensor's main job is to monitor catalyst efficiency, not control fuel management (although they can have a measure of influence over the front o2 sensor's operation, we'll just stick to basics for the sake of simplicity).
If you have a P0157, that doesn't mean your catalyst is bad (in fact, it usually never does). This code basically means that the Bank 2, Sensor 2 (or second o2 sensor back on bank 2) signal voltage is below what is considered normal for that sensor. It's a four wire sensor being supplied a 0.5 volt reference voltage (usually on the signal wire) and 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. This changing resistance affects the 0.5 volts on the signal/reference circuit. A heated o2 sensor is capable of switching between 0.1 volts and 0.9 volts. However, a properly working post-catalyst sensor will exhibit small changes at a slower rate than front (pre-catalyst) o2 sensors. It may switch slightly above and below 0.45 volts at the rate of about 1 switch per second or less(depends 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 is below a certain threshold for too long, P0157 will set.
Symptoms
Usually rear o2 sensor problems won't cause drivability issues since they are an input that measures catalyst efficiency (unlike front o2 sensors). However if you have other codes present, like lean codes or pre-o2 sensor codes, then they may cause drivability problems as well as P0143. The following symptoms may be present:
MIL (Malfunction Indicator Lamp) illumination Engine performance issues Engine running rough Engine running rich (only if PCM is trying to add fuel to compensate for low o2 sensor voltage reading) Engine running lean (only if vacuum leak is large enough that PCM can't compensate by adding fuel) Poor idleCauses
If you have reason to believe that your engine is running lean, fix that problem first, then clear codes and see if the P0157 code resets. An engine that is running lean could set this code, but usually there will be lean codes (P0171, P0174) present. The following could be causes:
Air leaks in exhaust in front of o2 sensor giving false reading Engine running excessively lean O2 sensor connector damaged/corroded O2 sensor signal circuit is shorted to ground O2 sensor ground circuit is open O2 sensor has failed (it may be contaminated with fuel or coolant) PCM has failedPossible Solutions
It's always good to start with checking for air leaks in the exhaust manifold and in the exhaust pipes. Fix these first.
With KOER (Key on engine running) use a scan tool or a voltmeter check the o2 sensor signal voltage with engine warm. You may have to increase the RPM to a fast idle to ch
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