So, either you have an engine performance issue or the (O2) sensor stopped working. When the mixture is lean, the sensor output voltage will drop down to about 0.2 volts or less. An o2 sensor (not Wideband o2) cannot accurately measure HOW rich or HOW lean the exhaust is. It will read very rich almost all the time in around the 0.845v range give or take a little but then if you … Since the sensor cannot read how rich or lean the mixture is, the computer continually increases and decreases injector pulse width (the amount of time the injector(s) is on, and so, how much fuel it injects) in an effort to … The downstream o2 sensor reads lean 0.1 V at idle and when driving the car it read 0.7 V. The upstream oscillate normally. Once the converter lights off, the downstream O2 sensor will settle down and “flat-line” at a steady voltage reading usually around 0.45 volts … Although theoretically the O2 sensor should cycle between 0.00 volts and 1.00 volts, in reality it cycles between 0.10 volts and 0.90 volts. It osculates its current usually between -.08 and +.4 at idle and it doesn’t really get to crazy but the downstream O2 sensor just seems funny to me. 0.500v is considered stoichmetric (the proper amount of fuel). When trouble code P0138 is set, this indicates that there is a high voltage (steadily above .9 volts) for more than 10 seconds indicating a lack of oxygen in the exhaust stream and an abundance of fuel at sensor 2 on the bank 1 of the engine. That's what you are reading on the front oxygen sensor. If you remove or disable your O2 sensor, you will not pass state vehicle emissions testing, as this modification is routinely used by those using their vehicles for off-road or track racing purposes. See attached schematic. The bank one portion of the code indicates that the issue is on the side of the engine that houses the number one … A GM O2 sensor signal stuck at 450 mV is an indication of an open O2 sensor circuit (signal wire) or faulty O2 signal ground. If an engine has suffered a recent head gasket failure, the O2 sensors may need to be replaced because of contamination. You will have to monitor the voltage supplied to the downstream oxygen sensor. Symptoms of a Bad O2 Sensor When an 02 sensor fails, there are a variety of diagnostic trouble codes (DTCs) that can pop up. Therefore, the downstream oxygen sensor (sensor 2) should produce a steady voltage of approximately 0.45 volts. Bad O2 sensors are one of those car problems that most drivers tend to postpone until more serious problems occur. These symptoms will rarely cause any immediate damage, but over time, if left unattended, they can cause damage to the engine and other systems mainly through improper air-fuel mixture. A bad oxygen sensor can impede the air/fuel mixture of an engine or interfere with the engine combustion, both of which can cause an engine to miss, or run irregularly. An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust. (254 vote (s)) If the signal from the downstream O2 sensor continues to mirror that from the upstream oxygen sensor(s), it means converter efficiency has dropped off and the converter isn’t cleaning up the pollutants in the exhaust. Hence, This means the sensor is operating properly. The sensor voltage should cycle or fluctuate within the 100 mV-900 mV (0.10 to 0.90V) range. Therefore, the downstream oxygen sensor (sensor 2) should produce a steady voltage of approximately 0.45 volts. It is sourced from the PCM (powertrain control module) and will probably be a pulsed voltage in the 5 - 8 volt range. If the sensor is sluggish or reads low, it needs to be replaced. Now after the catalytic converter, the exhaust should be cleaned up of carbon monoxide (CO) and voltage should stay close to … A good O2 sensor should show a lot of switching activity with readings bouncing back and forth from 0.3 volts or less to 0.8 to 0.9 volts. O2 sensors are damaged by the same things that damage catalytic convertors. Lead, manganese, and other heavy metals primarily. Silicates and acetates from some gasket-forming materials will also hurt them, but not so much catcons. The exhaust at that point should be switching from lean to rich and back (0.1v to 0.9v) around 60 times a minute. Meanwhile, if the catalytic converter is working properly, the downstream oxygen sensor should produce a fairly continuous reading of around 0.45 volts. Code P0138 is set when the downstream sensor sends too high of a voltage signal for an extended period of time. This is the voltage graph pattern of a brand new installed downstream O2sensor which shows the ideal pattern, a rich steady line with not much fluctuation. Once the converter lights off, the downstream O2 sensor will settle down and “flat-line” at a steady voltage reading usually around 0.45 volts or so. When the engine is first started, the downstream O2 sensor's output will mirror that of the upstream O2 sensor, and flip-flop back and forth between rich and lean. Why it reads lean at idle? If the (O2) sensor only produces a low or high voltage signal. In order for the catalytic converter to function correctly it needs a constant high and low of CO in the exhaust stream.
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