Oxy sensors

97 Blazer 135k miles, 4.3ltr. I want to start to replace the oxygen sensors. I want to replace just two for now because they are so expensive. Which two should be replaced before the other two? Right now I don't have any engine service light on, telling me there bad, just know it's probably time to change them for preventative maintance, thanks.

I'd say the forward ones closer to the engine but why if no code.
The ads all say every 50k but thats to sell you stuff - I have one with 175K, 140k, 135k, and 95k and never done one for failure except the heat shield on one rattling!

From what I understand, and heard, they have a lot to do with gas mileage. And now days, every little thing helps. Gas is $3.35 by me. Milwaukee, WI.

An oxygen sensor reacts to changes in fuel mixture in two ways: From lean to rich and from rich to lean. The faster the reaction time is, the faster the PCM can adjust the fuel mixture. The oxygen sensors reaction time slows down with miles. After 100,000 miles, reaction time has slowed substantially adversely affecting fuel mileage, performance and tail pipe emissions. That's why the factory recommends replacement at 100,000 miles.

Thanks for that explaination

Also if I may add a little more, you don't need to replace the 02 sensor behind/after the cat unless of course that sensor is causing the SES light to come on. This sensor does nothing for fuel management and only is put there to monitor the effeciency of the convertor itself. You only need to replace the sensors before the convertor.

Also if I may add a little more, you don't need to replace the 02 sensor behind/after the cat unless of course that sensor is causing the SES light to come on. This sensor does nothing for fuel management and only is put there to monitor the efficiency of the convector itself. You only need to replace the sensors before the convector.
While not as crucial as the precat sensors, the postcat sensor can have an effect on fuel management. To determine the converter efficiency, the PCM monitors the post cat sensor and compares that to the Bank 1 sensor. If it doesn't see activity within the right range from that sensor, it will adjust the fuel trim so that it can monitor the efficiency.

The diagnostic test for P0420 (low catalyst efficiency) is only run at idle after a long list of other tests run, and they all pass. The test might not run at all on a drive trip, but it can only run a maximum of one time per drive trip. A faulty downstream oxygen sensor will not affect fuel mileage due to the fact that the test is run in a matter of milliseconds, may or may not run each drive trip, and only one time when it does run.

When the test runs, the VCM momentarily commands a rich, followed by a lean mixture. The VCM then measures the time it takes for the downstream oxygen sensor to detect the change in fuel mixture. A longer time indicates higher catalyst efficiency. If the converter is hollowed out, or otherwise not operating efficiently, the time will be shorter. If the P0420 test fails one time, the DTC is set and the SES light illuminates. The VCM will turn off the SES light after 3 consecutive drive trips that the test has run and passed. The history DTC will clear if no fault occurs for 40 consecutive warm-up cycles.

Drive trip = Each time the vehicle is driven.
Warm-up cycle = Coolant temp rises more than 40F and reaches 158F during the same ignition cycle.

The diagnostic test for P0420 (low catalyst efficiency) is only run at idle after a long list of other tests run, and they all pass. The test might not run at all on a drive trip, but it can only run a maximum of one time per drive trip. A faulty downstream oxygen sensor will not affect fuel mileage due to the fact that the test is run in a matter of milliseconds, may or may not run each drive trip, and only one time when it does run.

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This is how I understand it too. The system is designed in that effeciency is checked through a test, and is not monitored continually. It will compare values with pre-cat sensors, yes, but it will do it through a test, and if the value is out of acceptable range, the system will throw a P0420. I never knew it will adjust fuel trim to bring effeciency within range, or try to anyway. I just thought it was a go, or no go.

I was refering to this quote about the catalyst monitor from the GM manual:

DESCRIPTION
In order to control the emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen.

OPERATION
The VCM has the ability to monitor this process by using the Bank 1 Sensor 1 and the Bank 1 Sensor 3 heated oxygen sensors. The Bank 1 Sensor 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The Bank 1 Sensor 3 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the ability of the catalyst to convert the exhaust gases efficiently. If the catalyst is operating efficiently, the Bank 1 Sensor 1 sensor will produce a far more active signal than that produced by the Bank 1 Sensor 3 sensor.

The HO2S sensors which are used in order to monitor the catalyst function are the same as the HO2S sensors which are used in order to control the fuel. Bank 1 Sensor 1 and Bank 1 Sensor 3 are used primarily for catalyst monitoring. These sensors also play a limited role in fuel control. If a sensor output indicates a voltage either above or below the 450 millivolt bias voltage for an extended period of time, the VCM will make a slight adjustment to the fuel trim in order to ensure that the fuel delivery is correct for catalyst monitoring.

Blazee,

That's interesting; my info is from the GM 1998 S/T shop manual. It explains the test a little differently than yours. It doesn't specify only using the bank 1 sensor 1 and bank 1 sensor 3. It does say the VCM watches for a lean or rich mixture just before it runs the test. If it detects a lean mixture, it will richen the mixture and then immediately lean the mixture, then measure the time for bank 1 sensor 3 to detect the change. If the detection time is too short, it sets the DTC. It's a cut and dried test, go or no go. It doesn't refer to it as “adjusting fuel trim”, but in actuality that’s what it’s doing. The injector duty cycle increases one time and decreases one time; a predetermined amount for a specified amount of time and the converter passes or fails. The converter is not given a “second chance” in the test.

I got the info from all data, which is suppose to have all the up to date info from the OEM manuals. My previous post was the info it shows for the sensor itself. Here is the info it shows for the P0420 code:

The system uses a three-way catalytic converter in order to control the emissions of Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The Vehicle Control Module (VCM) has the capability to monitor this process using the HO2S (Bank 1, Sensor 1). The HO2S (Bank 1, Sensor 3), located in the exhaust downstream of the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalysts ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the HO2S (Bank 1, Sensor 3) signal will be far less active than that produced by the HO2S (Bank 1, Sensor 1). If a problem exists which causes the VCM to detect an excessive HO2S (Bank 1, Sensor 3) activity outside of an acceptable range for an extended period of time, the VCM sets the DTC P0420. This DTC indicates that the three-way catalytic converters oxygen storage capacity is below a threshold considered acceptable.

That's great but it doesn't tell how, when or under what conditions the test runs or how it determines if the catalyst is above or below the threshold. That's where the GM shop manual comes in handy. It just goes to show you that you need both sources of information if you want the whole story.

It did have this stuff below and some flow charts but I only posted the summary:

CONDITIONS FOR SETTING THE DTC Converter Warm Up Test Enable

Engine in closed loop.
Commanded air fuel ratio equals 14.7.
The engine run time equals 346 seconds.
The Intake Air Temperature (IAT) is between -6.75-75°C (20-167°F).
The Engine Coolant Temperature (ECT) is between 75-117°C (167-243°F).
BARO equals 73 kPa.
The Predicted Catalyst Temperature is greater than or equal to 475°C (809°F).
The Throttle Position (TP) sensor is no more than 1.7%.
Engine idling at or near the recommended speed.
The Vehicle Speed Sensor (VSS) is no more than 1 mph.
No VSS DTCs.
No transmission DTCs.
No TP DTCs.
No purge DTCs.
No Idle Air Control (IAC) DTCs.
No HO2S DTCs.
No EVAP DTCs.
No misfire DTCs.
No EGR DTCs.
No ECT DTCs.
No Camshaft Position (CMP) DTCs.
No MAP DTCs.
No fuel trim DTCs.
No Ignition Control (IC) DTCs.
No IAT sensor DTCs.
No Mass Air Flow (MAF) sensor DTCs.ACTION TAKEN WHEN THE DTC SETS

The VCM will turn ON the Malfunction Indicator Lamp (MIL).

CONDITIONS FOR CLEARING THE MIL/DTC


The VCM turns the MIL OFF after 3 consecutive drive trips when the test has Run and Passed and not Failed.
A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during the same ignition cycle).
Use the scan tool Clear Information function.
Disconnect the VCM battery feed for 30 seconds.DIAGNOSTIC AIDS

Difficulty running the OBD II status DTC P0420 test may be encountered in areas where the test conditions cannot be maintained easily, especially in urban areas.

TEST DESCRIPTION

The numbers below refer to the step numbers on the diagnostic table.


Ensures that the fault is present.
Ensures that the knock sensor is capable of detecting detonation.

Yup, all that is in the GM manual, plus what the VCM actually does to perform the test. That's the long list I mentioned in post #8, and every one of those things has to pass and/or comply before the catalyst efficiency test runs. That's a lotta hoops to jump through, it's amazing the damn test EVER runs!

I replaced all four for about $200 bucks. Done for another 100K.