Oxygen Sensor I have a 1996 Aerostar and there's nothing in the maintenance manual about replacing the oxygen sensor. Shouldn't it be replaced regularly? My check engine light isn't on or any thing like that. Thank you.

This is a heated oxygen sensor and the computer is detecting that there is a problem with the heater portion not working. And the other code is caused because the heater is not working.

This could be a fuse, wiring, a connector with a poor connection, or the heater in the oxygen sensor (replace oxygen sensor in that case).

Generally oxygen sensors should be changed every 60,000 miles as a regular maintenance item. And when I replace mine, I get better gas mileage (also my engine is more lively). So I see this as a money saving thing myself.

But just about everyone I know will REFUSE to replace their oxygen sensors for some reason???

Anyway information for troubleshooting and/or replacing this would probably be in an aftermarket repair manual for your vehicle (auto parts store). But this information would for sure be in a Factory Service Manual set for your vehicle which you can order from your dealer.

There is no way to display trouble codes without a scan tool of some kind. Being a 1996 it uses OBD II protocol which means you can find a generic scan tool that will retrieve codes at most parts stores. Generally they will scan the codes for you for free. Just be careful about taking the parts persons advice as far as what it will take to fix the problem. Many times a parts person will recommend replacing whatever part the code says but that is often not the correct fix. For instance a code P0171 indicates an oxygen sensor is reading lean so a lot of times people will replace an oxygen sensor for that but it's rarely caused by a bad sensor . Typically it's caused by a vacuum leak or a bad mass air flow sensor or sometimes a weak fuel pump. I guess what I'm getting at is make sure you fully diagnose the vehicle. Knowing the trouble code is just the beginning there can be several causes for any given code.

That is a low signal error from the right bank upstream oxygen sensor. In most cases, it is caused by a weak oxygen sensor. Before replacing the oxygen sensor, double check fuel pressure, volume and quality. A low signal from the oxygen sensor can also be caused by a lean fuel condition.

You have two oxygen sensors but 21 is upstream sensor. This is the sensor between engine and catalytic converter. There is three possible problems that it could be.
1 oxygen sensor input voltage maintained above the normal operating range.
2 no signal at 02 2/1 sensor
3 oxygen sensor input voltage sustained above thenormal operating range. What all this means is you need to replace the sensor.
when getting the replacement this is your O2 2/1 sensor.

Here are some sensors.http://www.autozone.com/autozone/parts/1996-Subaru-Legacy/Oxygen-Sensor/_/N-j4zs5Z8gcvp

This may have 2 sensors since it's a v6.

An O2 Sensor Code, does not mean the sensor is bad.
You failed for the check light, not the code.
The sensor could be old and worn out,when ever you get a check light,yes, you do retrieve all codes first. Then the code is used to direct you to an area or problem, within the powertrain .
O2 Codes are caused more by vacuum issues or other issues,and the code is set, as a result of trying to correct short term fuel trim,and goes out of range,setting the light/code

Your car will run rough like sand paper. The oxygen sensor lets the car know how lean or rich it needs the fuel flow to be.

A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• An oxygen sensor is not reading (functioning) properly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings

Some suggested steps for troubleshooting a P0420 code include:

• Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
• Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
• Inspect the downstream heated oxygen sensor (HO2), replace if necessary
• Replace the catalytic converter

you need to borrow the oxygen sensor wrench from autozone or whereever you bought the sensor. Section 03-14: Engine Controls, Electronic 1996 Taurus, Sable Workshop Manual REMOVAL AND INSTALLATION -------------------------------------------------------------------------------- Heated Oxygen Sensor (HO2S) SPECIAL SERVICE TOOL(S) REQUIRED Description Tool Number Oxygen Sensor Wrench T94P-9472-A Removal and Installation Disconnect battery ground cable (14301). Refer to Section 14-01 . On 3.0L (4V) engine remove cowl extension and right side of leaf screen. Disconnect engine control sensor wiring (12A581) from heated oxygen sensors (HO2S) (9F472). Do not remove harness connectors from the brackets. Raise vehicle on hoist. Refer to Section 00-02 . NOTE: If excessive force is needed to remove a heated oxygen sensor, lubricate the heated oxygen sensor with penetrating oil prior to removal. Remove heated oxygen sensors from exhaust manifolds (9430) using Oxygen Sensor Wrench T94P-9472-A. To install, reverse Removal procedure. Apply a coating of anti-sieze compound to threads of heated oxygen sensor. Tighten heated oxygen sensors to 34-46 Nm (26-34 lb-ft).