What should downstream o2 sensor read

There are other sensors to monitor these factors, too, but the O2 sensor is the master monitor for what's happening with the fuel mixture. Consequently, any problems with the O2 sensor can throw the whole system out of whack. The computer uses the oxygen sensor's input to regulate the fuel mixture, which is referred to as the fuel "feedback control loop. This produces a corresponding change in the O2 sensor reading. This is referred to as "closed loop" operation because the computer is using the O2 sensor's input to regulate the fuel mixture.

The result is a constant flip-flop back and forth from rich to lean which allows the catalytic converter to operate at peak efficiency while keeping the average overall fuel mixture in proper balance to minimize emissions. It's a complicated setup but it works. When no signal is received from the O2 sensor, as is the case when a cold engine is first started or the 02 sensor fails , the computer orders a fixed unchanging rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to regulate the fuel mixture.

If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions.

A bad coolant sensor can also prevent the system from going into closed loop because the computer also considers engine coolant temperature when deciding whether or not to go into closed loop. The O2 sensor works like a miniature generator and produces its own voltage when it gets hot. Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb. The bulb is coated on the outside with a porous layer of platinum. Inside the bulb are two strips of platinum that serve as electrodes or contacts.

The outside of the bulb is exposed to the hot gases in the exhaust while the inside of the bulb is vented internally through the sensor body to the outside atmosphere. Older style oxygen sensors actually have a small hole in the body shell so air can enter the sensor, but newer style O2 sensors "breathe" through their wire connectors and have no vent hole.

It's hard to believe, but the tiny amount of space between the insulation and wire provides enough room for air to seep into the sensor for this reason, grease should never be used on O2 sensor connectors because it can block the flow of air. Venting the sensor through the wires rather than with a hole in the body reduces the risk of dirt or water contamination that could foul the sensor from the inside and cause it to fail.

The difference in oxygen levels between the exhaust and outside air within the sensor causes voltage to flow through the ceramic bulb. The greater the difference, the higher the voltage reading. An oxygen sensor will typically generate up to about 0. When the mixture is lean, the sensor's output voltage will drop down to about 0.

When the computer receives a rich signal high voltage from the O2 sensor, it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean low voltage , the computer reverses again making the fuel mixture go rich.

This constant flip-flopping back and forth of the fuel mixture occurs with different speeds depending on the fuel system. The transition rate is slowest on engines with feedback carburetors, typically once per second at rpm. Engines with throttle body injection are somewhat faster 2 to 3 times per second at rpm , while engines with multiport injection are the fastest 5 to 7 times per second at rpm. The oxygen sensor must be hot about degrees or higher before it will start to generate a voltage signal, so many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly.

The heating element can also prevent the sensor from cooling off too much during prolonged idle, which would cause the system to revert to open loop. Heated O2 sensors are used mostly in newer vehicles and typically have 3 or 4 wires. Older single wire O2 sensors do not have heaters.

This causes a big jump in fuel consumption as well as emissions. And if the converter overheats because of the rich mixture, it may suffer damage. These are the OBD codes associated with O2 sensor faults:. If an O2 sensor is marginally sluggish or is slightly biased rich or lean, it may not set a fault code. You can read the O2 sensor's voltage output with a scan tool or digital voltmeter, but the transitions are hard to see because the numbers jump around so much. A scope will display the sensor voltage output as a wavy line that shows both it's amplitude minimum and maximum voltage as well as its frequency transition rate from rich to lean.

A good O2 sensor should produce an oscillating waveform at idle that makes voltage transitions from near minimum 0. Making the fuel mixture artificially rich by feeding propane into the intake manifold should cause the sensor to respond almost immediately within milliseconds and go to maximum 0. Creating a lean mixture by opening a vacuum line should cause the sensor output to drop to its minimum 0. If the sensor does not flip-flop back and forth quickly enough, it may indicate a need for replacement.

If the O2 sensor circuit opens, shorts or goes out of range, it may set a fault code and illuminate the Check Engine or Malfunction Indicator Lamp. If additional diagnosis reveals the sensor is defective, replacement is required. But many O2 sensors that are badly degraded continue to work well enough not to set a fault code, but not well enough to prevent an increase in emissions and fuel consumption.

The absence of a fault code or warning lamp, therefore, does not mean the O2 sensor is functioning properly. The sensor may be lazy, or biased rich or lean. A company called Lenehan Research makes a handheld O2 sensor tester that checks the response time of the O2 sensor to show if it is good or bad. The tester requires the oxygen sensor to jump from below mV to above mV in less than mS when the throttle is snapped.

If the sensor does not respond quickly enough it fails the test. The tester also shows closed loop operation on a fast, ultra-bright, colored 10 LED display, and tests the PCM control of the fuel feedback control system.

Any O2 sensor that is defective obviously needs to be replaced. But there may also be benefits to replacing the O2 sensor periodically for preventative maintenance. Replacing an aging O2 sensor that has become sluggish can restore peak fuel efficiency, minimize exhaust emissions and prolong the life of the converter. Unheated 1 or 2 wire wire O2 sensors on through early s vehicles can be replaced every 30, to 50, miles.

Heated 3 and 4-wire O2 sensors on mids through mids applications can be changed every 60, miles. The oxygen sensor can be removed from the exhaust manifold using a special oxygen sensor socket which has a cutout to clear the wires , or a 22mm socket.

The sensor will come out easier if the engine is slightly warm but not hot to the touch. Place the socket over the sensor and turn counterclockwise to loosen it.

If it is frozen, apply penetrating oil and heat around the base of the sensor. When installing a new "direct fit" or OEM oxygen sensor, the wiring connector on the new sensor will plug into the connector with no modifications needed. But if you are installing a "universal" oxygen sensor, the original wiring connector will have to be cut off so the wires on the new sensor can be spliced to the wires that went to the connector.

With 4-wire sensors, one wire is the signal wire, one is ground, and the other two are for the heater circuit. The wires are color coded, but the colors on the universal sensor probably won't match those on the original sensor.

See the chart below from the color coding used on various brands of oxygen sensors:. It depends on the model year and type of engine. On most four and straight six cylinder engines, there is usually a single oxygen sensor mounted in the exhaust manifold.

On V6, V8 and V10 engines, there are usually two oxygen sensors, one in each exhaust manifold. On later model vehicles with OBD II some and '94 models, and all and newer models , one or two additional oxygen sensors are also mounted in or behind the catalytic converter to monitor converter efficiency. These are referred to as the downstream O2 sensors, and thee will be one for each converter if the engine has dual exhausts with separate converters.

When displayed on a scan tool, the right and left upstream oxygen sensors are typically labeled Bank 1, Sensor 1 and Bank 2, Sensor 1. The Bank 1 sensor will always be on the same side of a V6 or V8 engine as cylinder number one. On a scan tool, the downstream sensor on a four or straight six cylinder engine with single exhaust is typically labeled Bank 1, Sensor 2. Or, the downstream oxygen sensor might be labeled Bank 1 Sensor 3 if the engine has two upstream oxygen sensors in the exhaust manifold some do to more accurately monitor emissions.

It's important to know how the O2 sensors are identified because a diagnostic trouble code that indicates a faulty O2 sensor requires a specific sensor to be replaced. Bank 1 Sensor 1 might be the back O2 sensor on a transverse V6, or it might be the one on the front exhaust manifold. What's more, the O2 sensors on a transverse engine might be labeled differently than those on a rear-wheel drive application.

There is not a lot of consistency as from one vehicle manufacturer to another as to how O2 sensors are labeled, so always refer to the OEM service literature to find out which sensor is Bank 1 Sensor 1 and which one is Bank 2 Sensor 1. All Rights Reserved. Smog questions might be posted on our online forum to assist other vehicle owners. No personal information is published. Which Type Should I Choose?

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