In my experience O2 sensors slowly go bad and to the best of my knowledge, these haven't been replaced at 210K miles. Question, do I need to replace all 4 or just the front 2? FWIW, I am not getting any codes, but MPG is poor even for a power tune. They are called upstream and downstream. I assume front ones are the upstream.
210K miles on 2008 Tahoe. Replacing O2 sensors
- Thread starter pacem
- Start date
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wjburken
Supporting Member
I would recommend replacing all 4 sensors.
Yes, Upstream is before the Cats, Downstream is after the Cats.
You may also have a Cat failing as well.
wjburken
Supporting Member
Yes. You will need qty 2 of each.
You are right about both upstream being the ones before the cats, left and right. Time was when Oxygen sensors were recommended for replacement at 100k miles. The fronts are monitoring the exhaust mixture from the engine and the rears monitor the catalytic converter efficiency. Lazy/failing fronts will change help air/fuel mixture and therefore fuel mileage is effected.
trailblazer
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Is there a definitive way to know if they have failed? I’d like to know this before I just start replacing them.
This should keep you busy for a while: Keep this in mind though, it can test good now and your replace 1 or 2 sensors to save money and then next week, because of the sensor age, the other 1 or 2 can go bad.
Learn How to Check an Oxygen Sensor
Learning how to check an oxygen sensor (O2) will help you confirm a potential problem with the sensor in your vehicle before you spend time and money replacing a unit that might not even need a replacement in the first place. Oxygen sensors detect the amount of oxygen content in the exhaust stream and turn this information into a voltage signal that your car computer uses to control fuel ratio and emissions.
However, whenever the sensor’s signal changes—and stays out of—its normal operating parameters, it causes the computer to store a trouble code in memory and turn a malfunction indicator light (MIL) or Check Engine light on your dashboard to alert you of the problem.
Still, whether you suspect a bad oxygen sensor or a computer trouble code points to a potential issue with the O2 sensor, the problem may lay somewhere else. All the computer does is inform you where the problem was detected. For example, you may have a loose or torn vacuum hose, causing the sensor to read a constant high rate of oxygen in the exhaust stream; or the sensor electrical connector may have become loose, preventing the device from working. Instead, the computer detects a sensor abnormal operation.
So before replacing the unit, you can use the sensor’s operating characteristics to verify whether you actually need to replace it.
Connecting Your Voltmeter to the Oxygen Sensor
To run this test, you will need a 10-megaohm impedance digital voltmeter. Most digital voltmeters come with a 10-megaohm protection to prevent the meter from drawing too much electrical current and damaging electrical or electronic components during a test.
Also, before you start your tests, locate the oxygen sensor you want to troubleshoot. On pre-1996 vehicle models, you'll find the sensor usually on or near the exhaust manifold. On 1996 and newer models, you'll see a sensor near the exhaust manifold, and another one near the catalytic converter. However, some vehicle models have up to five or more sensors. Make sure you know which sensor you need to check.
When you retrieve diagnostic trouble codes (DTC) from your car computer, you may also get information about the specific sensor at fault, depending on your scan tool features. For example, you may get a Bank I, sensor 1 at fault, which points to the O2 sensor on or near the exhaust manifold on the cylinder head that contains cylinder number 1. A bank I, sensor 2, points to the sensor on the same side but farther down the exhaust system, probably right before or after the catalytic converter. The same goes for the other cylinder head—on V type engines—which is considered Bank II.
To locate Bank I and Bank II, consult your vehicle service manual, if necessary.
Then, if the sensor you're testing has more than one wire (heated sensor), locate the signal wire by consulting your vehicle service manual, if necessary.
aftermarket manual for your specific vehicle make and model at your local auto parts store or online.
Another option is to pierce the sensor's signal wire through the insulation with a pin and connect the meter probe to the pin. But keep the pin from touching ground.
If you decide to go with the latter method, after you finish your tests remove the pin and cover the pierced section of wire with electrical tape to prevent moisture and corrosion from creeping into the wire.
Reading the Oxygen Sensor’s Signals
Start the engine and check the sensor’s voltage signals on your voltmeter. The sensor voltage should cycle or fluctuate within the 100 mV-900 mV (0.10 to 0.90V) range approximately. This means the sensor is operating properly.
If the O2 sensor only produces a low- or high-voltage signal, either you have an engine performance issue or the oxygen sensor stopped working. To verify sensor operation, conduct the next two tests.
Test the Oxygen Sensor Response to a Lean Fuel Condition
Learn How to Check an Oxygen Sensor
Learning how to check an oxygen sensor (O2) will help you confirm a potential problem with the sensor in your vehicle before you spend time and money replacing a unit that might not even need a replacement in the first place. Oxygen sensors detect the amount of oxygen content in the exhaust stream and turn this information into a voltage signal that your car computer uses to control fuel ratio and emissions.
However, whenever the sensor’s signal changes—and stays out of—its normal operating parameters, it causes the computer to store a trouble code in memory and turn a malfunction indicator light (MIL) or Check Engine light on your dashboard to alert you of the problem.
Still, whether you suspect a bad oxygen sensor or a computer trouble code points to a potential issue with the O2 sensor, the problem may lay somewhere else. All the computer does is inform you where the problem was detected. For example, you may have a loose or torn vacuum hose, causing the sensor to read a constant high rate of oxygen in the exhaust stream; or the sensor electrical connector may have become loose, preventing the device from working. Instead, the computer detects a sensor abnormal operation.
So before replacing the unit, you can use the sensor’s operating characteristics to verify whether you actually need to replace it.
Connecting Your Voltmeter to the Oxygen Sensor
To run this test, you will need a 10-megaohm impedance digital voltmeter. Most digital voltmeters come with a 10-megaohm protection to prevent the meter from drawing too much electrical current and damaging electrical or electronic components during a test.
Also, before you start your tests, locate the oxygen sensor you want to troubleshoot. On pre-1996 vehicle models, you'll find the sensor usually on or near the exhaust manifold. On 1996 and newer models, you'll see a sensor near the exhaust manifold, and another one near the catalytic converter. However, some vehicle models have up to five or more sensors. Make sure you know which sensor you need to check.
When you retrieve diagnostic trouble codes (DTC) from your car computer, you may also get information about the specific sensor at fault, depending on your scan tool features. For example, you may get a Bank I, sensor 1 at fault, which points to the O2 sensor on or near the exhaust manifold on the cylinder head that contains cylinder number 1. A bank I, sensor 2, points to the sensor on the same side but farther down the exhaust system, probably right before or after the catalytic converter. The same goes for the other cylinder head—on V type engines—which is considered Bank II.
To locate Bank I and Bank II, consult your vehicle service manual, if necessary.
Then, if the sensor you're testing has more than one wire (heated sensor), locate the signal wire by consulting your vehicle service manual, if necessary.
- A single-wire sensor uses that wire as the signal wire.
- Two-wire sensors use one wire for the sensor's signal and the other one to power the heater.
- Three-wire models, use one wire for the signal and the other two wires to power and ground the heater.
- Four-wire sensors, though, use one of the wires to ground the sensor itself.
- Once you have the appropriate voltmeter on hand and have located the sensor, warm up your car engine to operating temperature. You can do this by taking your car for a 20-minute drive on the highway or idling the engine for about 15 to 20 minutes at a fast idle speed.
- Turn off the engine and set your voltmeter to the mV (millivolt) DC scale.
- If you are testing an O2 sensor near the catalytic converter, lift your vehicle using a floor jack and safely support the vehicle on a couple of jack stands and block the rear wheels.
- Be careful when connecting your meter. When the engine is at operating temperature, the exhaust manifold and pipes are extremely hot. Don't burn yourself and keep your meter and probes away from hot surfaces.
- On sensors with one to three wires, connect the meter’s red probe to the sensor's signal wire and the meter’s black probe to a good ground on your engine. On four-wire sensors, connect the meter's black probe to the sensor's ground wire. If necessary, consult the wiring diagram in your vehicle repair manual.
Another option is to pierce the sensor's signal wire through the insulation with a pin and connect the meter probe to the pin. But keep the pin from touching ground.
If you decide to go with the latter method, after you finish your tests remove the pin and cover the pierced section of wire with electrical tape to prevent moisture and corrosion from creeping into the wire.
Reading the Oxygen Sensor’s Signals
Start the engine and check the sensor’s voltage signals on your voltmeter. The sensor voltage should cycle or fluctuate within the 100 mV-900 mV (0.10 to 0.90V) range approximately. This means the sensor is operating properly.
If the O2 sensor only produces a low- or high-voltage signal, either you have an engine performance issue or the oxygen sensor stopped working. To verify sensor operation, conduct the next two tests.
Test the Oxygen Sensor Response to a Lean Fuel Condition
- First, disconnect the hose from the positive crankcase ventilation (PCV) valve leading to the intake manifold. This will allow more air to enter the engine. If you need to locate the PVC valve, consult your vehicle service manual.
- Check the sensor’s signal voltmeter reading. An oxygen sensor interprets an increase in oxygen as a fuel lean condition and emits a signal close to 200mV (0.20 V). If the sensor doesn't respond accordingly or takes time to respond, the sensor isn't working properly.
- Reconnect the hose to the PCV valve.
- Disconnect the plastic duct from the air cleaner assembly on your vehicle.
- Block the duct opening leading to the engine with a clean rag. This will reduce the amount of air going into the engine.
- Check the sensor’s signal voltmeter reading. An oxygen sensor interprets a decrease in oxygen as a rich fuel condition and emits a signal close to 800mV (0.80 V). If the sensor doesn't respond accordingly or takes time to respond, the sensor isn't working properly.
- Reconnect the air duct to the air cleaner assembly and turn off the engine.
trailblazer
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So if I never get a MIL regarding the sensors, it’s safe to say they are fine for now? If I were to get a MIL I’d probably replace them all.
