what are oxygen sensors supposed to read in graph obd scanner

The following article is derived from my book "Diagnostics Strategies of Modern Automotive Systems - Sensor Testing". Here I don't become into details of O2 sensor construction, equally it's more of an education into testing and diagnosing it. Feel free to call me with any questions at the number found here. Enjoy...

The O2 or Oxygen sensor works like a tiny voltage generator. Information technology really produces a voltage in the theoretical range of 0.01 to 0.98 volts. It does this depending on the Oxygen content of the frazzle. This signal is a major input to the ECM, which it uses to control the air-fuel mixture and emissions.

Theory of Functioning

The O2 sensor measures the oxygen content of the exhaust. The O2 sensor's sensing power comes about by producing a small voltage proportionate to the exhaust oxygen content. In other words, if the oxygen content is low it produces a high voltage (0.90 Volts - Rich mixture) and if the oxygen content is loftier it produces a low voltage (0.10 Volts - Lean mixture). Although theoretically the O2 sensor should cycle between 0.00 volts and one.00 volts, in reality information technology cycles between 0.10 volts and 0.90 volts.

In many modern cars the O2 sensor has been replaced by the AFR or Wide-Band sensor. Only, the rear or after-true cat O2 sensor is still the same quondam fashioned O2 sensor.

A few key bug are very of import in the analysis of O2 sensor signals.

• An O2 sensor volition cycle between 0.x to 0.90 or almost 1 volt.

• An O2 sensor has to attain the 0.8x Volts amplitude marking while at full performance.

• An O2 sensor also has to accomplish the 0.1x Volts amplitude mark while at total operation.

(Full performance means the engine is fully warmed up, O2 sensor higher up the 600 deg. F. operating temperature, and no fuel or mechanical problems present. As well well-nigh rear O2 sensor will not wheel as broad, but during testing you have to goose the throttle and practice a few WOT events to prove your O2 sensor voltage swing.)

• The forepart O2 sensor must cycle at least once per second, which would bear witness 3 cantankerous counts on the scan tool PID.

• Silicone is the leading cause of O2 contamination.

• Information technology is easier for an O2 sensor to go from rich to lean than vise-versa.

• O2 sensors tend to fail on rich bias. In other words, they tend to shift their cycling to the upper side or rich side of the voltage scale.

• There is a small pigsty at the torso of the O2 sensor, which allows it sample the outside reference Oxygen.

• Contrary to what many people think, an O2 sensor WILL Not bike past itself. The O2 sensor bike is a direct effect of the ECM response to the changes in the mixture.

• When the O2 cycles and crosses the 0.450 volts mark every 2nd, the organisation is in Close-LOOP.

• Even though an O2 sensor is cycling and crossing 0.450 volts (ECM in shut loop) information technology DOES Not mean that information technology is working properly.

• O2 sensor operation is extremely of import not only to keep HC & CO emissions low but also to reduce NOx as well.

• Proper O2 sensor cycling will determine the catalytic converter's efficiency. The catalytic converter needs the O2 sensor cycling at its proper amplitude and frequency for it to function at its maximum efficiency.

• An O2 sensor with a high voltage reading does not necessarily mean that the mixture is rich or high in fuel content. An EGR valve problem will send the O2 signal loftier equally well.

• The O2 sensor signal stuck at 450 mV is an indication of an open up O2 sensor circuit (indicate wire) or faulty O2 bespeak ground. The 450 mV value is called a bias voltage and it is non the same for all manufacturers. Some manufacturers employ a defended O2 sensor ground. Such a ground lead is fastened to the engine block or chassis and feeds an ECM O2 footing pin only. The O2 circuit is and so grounded through the inside of the ECM electronic board by this basis wire. A loss of this footing would likewise put the O2 sensor signal at around 450 mV, which makes it await similar an open excursion. The same holds true for Chrysler, but these use a different O2 bias voltage, which is usually two.00 to iv.00 volts. Recollect, this bias voltage circuit is very low electric current.

A large misconception among technicians trying to sympathise O2 sensors is that they cycle by themselves. The O2 sensor just reads oxygen content in the exhaust, THAT'S It. Excess oxygen in the form of regular ambient air volition send the O2 sensor voltage indicate low (under 0.450 volts) and lack of it will transport the voltage signal loftier (over 0.450 volts). A stuck open EGR valve will create a lack of oxygen in the exhaust, since the re-circulating exhaust has all its oxygen already burnt . The ECM sometimes uses the O2 sensor to cheque for proper EGR operation and sets a code if necessary. So, be aware of the fact that a vehicle might be running lean because the ECM sees a rich O2 signal due to a defective (stuck open) EGR valve. Since the ECM sees a rich bespeak, information technology will endeavor to right with a lean command and endeavor to lower the O2 sensor's high voltage point. Newer non-EGR systems don't take this issue, since the EGR effect is done through variable valve timing. However, an event with the variable valve timing overlap will create the same consequence as a stuck open EGR valve.

Atmospheric condition that Affect Operation

Notation: WHEN PERFORMING O2 SENSOR CHECKS, IT IS IMPORTANT TO TAKE MEASUREMENTS AT IDLE AND 2000 RPM. BE Enlightened THAT O2 SENSOR PRE-CONDITIONING IS IMPORTANT, Fifty-fifty ON THE NEWER STYLED HEATED O2 SENSORS (Non AFR sensors). PRE-Status THE O2 SENSOR BY RAISING THE ENGINE SPEED TO 2000 RPM FOR Virtually 15 SECONDS OR SO. THE O2 SENSOR HAS TO BE Above 600 º F. TO BE ABLE TO OPERATE PROPERLY. LONG PERIODS OF IDLE TIME Tin RENDER A NON-HEATED OR OLDER O2 SENSOR TOO COLD FOR IT TO FUNCTION AT ALL. AT THE SAME TIME, Do Non TRY TO FORCE A HEATED O2 SENSOR INTO OPERATION. AN O2 SENSOR WITH A FAULTY HEATER Volition Go INTO CLOSED-LOOP Later on A Proficient WARM-UP SESSION.

After an engine has ran through its warm upwardly period (O2 sensor has no upshot on engine operation while the engine is cold), the ECM and then looks for the O2 value. The 0.450 volts mark is considered almost universally as the midway point or crossover point for O2 sensor operation. If the signal is on the rich side (above 0.45 volts), and then the ECM will answer with a lean command (reducing injector pulsation), or if the indicate is on the lean side (below 0.45 volts) then the ECM

volition answer with a rich command (increasing injector pulsation). The corporeality of injector pulse correction is proportional to the voltage seen by the ECM at the O2 sensor signal wire. The higher the voltage the more the ECM reduces on-time to the injector. The lower the voltage the more than the ECM increases the injector on-time. The ECM is constantly doing exactly but that, slightly increasing and decreasing injector pulsation. The constant adjustment is what gives the O2 sensorsignal the switching advent (sine wave) on the telescopic screen.

NOTE: The ECM's fuel pulse corrections performed constantly to the injector signal is chosen Curt TERM FUEL TRIM ( GM called it INTEGRATOR ) and LONG TERM FUEL TRIM ( GM chosen it BLOCK Acquire ) on the scanner. FUEL TRIMS is the system'south departure of the BASE-INJECTION pulse. Analyzing LTFT and STFT is a great way to know a particular vehicle's fuel consumption tendency or how well that vehicle has been performing with regards to fuel command. STFT and LTFT is the first thing to look for when assessing fuel control problems.

The fact that the O2 sensor signal is switching rich-lean-rich-lean also reveals that the ECM is decision-making the injector pulsation and therefore that the system is in shut loop style. An ECM in full control (O2 sensor cycling) is said to be in close loop because of the shut-excursion action of O2 sensor-to ECM-to injector pulse command then to O2 sensor and back to the ECM. The ECM must be in command at all times except during warm upwardly, WOT, power enrichment, and deceleration mode.

The O2 sensor not only has to cycle, it also has to bike fast enough (proper frequency) and wide enough (proper amplitude). At least one cycle per second (1 Hz) must exist seen at the signal wire in order for the O2 to exist considered practiced (not lazy). A 1 cycle per 2d will make the scope trace get beyond the 0.450 volts mark approximately three times, which the ECM recognizes as iii cantankerous counts. A irksome O2 sensor will have a dissentious consequence on the catalytic converter and release excessive amounts of emissions to the atmosphere.

A bike are the complete rich and lean crests of the O2 sensor signal, while crossing the 0.45 voltage point. Proper amplitude refers to the O2 sensor's power to reach full rich ( 0.90 volts ) and full lean ( 0.10 volts ) when cycling. The college the voltage seen at the O2 betoken line the more than the ECM reduces pulsation to the injectors. The lower the voltage seen at the O2 signal line the more the ECM increases injector pulsation. This is the reason why an O2 sensor that is not reading the mixture properly, at total amplitude and frequency, volition actually misguide the ECM into a wrong fuel control pattern. Once the O2 sensor has reached its correct temperature of 600 º F, look for an O2 signal cycle with the correct amplitude and frequency and information technology will surely indicate a perfectly operating O2 sensor.

Component Testing

Annotation: On early on OBD II systems, the post catalytic converter O2 sensor has no outcome on fuel control. The mail service catalytic O2 sensor was originally responsible for but monitoring catalytic converter efficiency. On well-nigh systems, the post converter O2 sensor indicate should never mimic or follow the pre-cat O2 signal. That would betoken a defective or low oxygen storage capability at the converter . On early OBD II systems, the postal service-cat O2 sensor should show piffling or no voltage fluctuations on a telescopic waveform, since all the mixture fluctuations are being absorbed by the catalytic converter.

Stating around model twelvemonth 1999, a new blazon of converter came on the marketplace, called "Low Oxygen Storage Converter" or LOC. With an LOC, the pre and mail service O2 sensors cycle at the same rate. These converters are tested by measuring the lag-time between the 2 signals. A further development of this system is that the mail service converter signal is too used for A/F correction, only to a less extent.

These simple steps should be followed whenever testing O2 sensors.

1. Scan the vehicle for any O2 sensor codes and analyze the data stream PID. O2 sensor voltage should cycle normally with proper amplitude and frequency. An O2 sensor stuck at a fixed bias voltage is an indication of an open O2 excursion or lack of O2 sensor (dedicated) ground. If possible use a graphing multi-meter to analyze the O2 sensor data to decide whatever possible problems.

2. While reading the scan values, goose the throttle and observe for O2 sensor minimum and maximum values (0.1x volts to 0.9x volts). Although this is not a conclusive evidence of right O2 sensor performance, it serves equally a preliminary indication of proper operation.

3. Some automotive manufacturers utilize a dedicated O2 sensor ground wire that is grounded somewhere at the engine block or chassis. A loss or rupture of this ground wire volition render the O2 sensor useless. This ground wire feeds simply the ECM's O2 sensor circuit. The main engine basis does not feed this blazon of O2 sensor excursion.

4. Verify the O2 sensor wire integrity. About O2 sensors are biased and an open signal wire will give a reading of any the bias voltage is. Later on model Jeep/Chrysler O2 circuits tend to be biased at around 2 or 4 volts, therefore, a constant reading of around 2 or 4 volts on a Chrysler is too an indication of an open circuit. In many of these cases, the ECM will put an "O2 sensor High Voltage" code.

5. Finally, verify for correct O2 sensor performance with a scope or graphing multi-meter. Bank check for proper amplitude and frequency. Recall that the scanner O2 sensor readings are only interpreted values and may not testify the real voltage reading. This is the reason for doing this final manual exam.

Hope you enjoyed this article on testing O2 sensors. It is largely based on one of my publications and fabricated bachelor hither on LinkedIn. For more in depth data on automotive technology visit u.s.a. on our YouTube Channel (ADPTraining), DIYCarDoctor and Automotive-Diagnostics-Publishing. Thank you...Mandy.

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Source: https://www.linkedin.com/pulse/how-test-oxygen-sensor-mandy-concepcion