'04 Tribute, 62K
Check engine light came on, code (forgot #) was the rear oxygen sensor, behind cat. Bought one from a big box parts place, Bosch, 15718. Took the car to another big parts place that does repairs. They installed my sensor. A week or two later, light comes back on. Eventually I take it back there and they say it checks out fine. Light goes off. They do mention that if it comes on again, I may need to get the sensor from the dealer because it may need some type of heat exchange (?) thing. Light came on as I left parts place parking lot.

Suggestions?

 

So the rear O2 sensor sets the check engine light on, big deal. This does not mean it is kaput; in the majority of cases it just means it is doing the job it was intended to do, i.e. telling you there's something wrong somewhere with your engine such that its ability to pass emission standards has been compromised: that can mean anything between the air intake right through to and including the catalytic converter in the exhaust system.
Unless you want to be nickeled and dimed out of a mass airflow sensor in front end, a catalytic converter at the back end, and everything in between (at an average cost of $200 a pop), it would be a really good idea if you made sure to at least write down those fault code numbers and the EXACT NAME of the item(s) that people suggest might be causing you grief before trying to get help on this forum.

P.S. I think 62K is too soon for a rear O2 sensor to throw in the towel, so there was quite probably nothing wrong with it in the first place.

 

Thanks. I believe the code was:
P1136 HO2S Bank 1 Sensor 1 Heater Circuit High Input

3 different places mentioned the rear Oxygen sensor, and I didn't know any better.
My concern is a cat convertor going bad and costing me tons of money. Still not sure where to go with this.

 

OK, good; now let's get things sorted out.
1. DTC P1136 means the heater circuit voltage of the bank 1 FRONT O2 sensor is high, so you've been messing with the wrong sensor; at least you can set your mind at rest about the catalytic converter because a bad cat may affect the REAR O2 sensor, and probably not its heater circuit either for that matter.
2. The O2 sensor heater circuits are there to activate the O2 sensor such that the sensitive element that does the actual measurement is in the optimum temperature range to do its job.
3. The O2 sensor heating is done by a voltage originating from the ignition switch which finds its way to one end of the heater element on terminal C of the O2 sensor.
The other end of the heater in the sensor is on terminal D. Terminal D is connected back to the PCM (power control module/ ECU, or whatever) where a switching semiconductor in series with the D terminal intermittently makes a circuit to ground so as to regulate the temperature of the O2 sensor at a constant value regardless of the temperature of the exhaust gas.
Your P1136 issue is due to the voltage across the C and D terminals of the sensor being abnormally high; this can be because the sensor is defective, the sensor connector or car wiring on the D terminal has a short to ground, or the PCM or its connector is defective, and for whatever reason, the heater current is always ON.
Now I can't swear by these values, because they come from my Protege shop manual, but for what they're worth, the resistance measured between the C and D terminals of an O2 sensor (unplugged) having a good heater, should be about 15 7 ohms if your car is compliant with federal emission standards, or about 5.6 ohms if compliant with California emission standards.
If the front O2 sensor heater circuit is screwed up, the sensor will not operate in its proper range; as a result your fuel/ air mixture ratio cannot be properly controlled, hence the CEL comes on because of a perceived lean or rich mixture seen downstream of the cat. by the rear O2 sensor.
You should sort this out soon because a continuously rich mixture makes the cat. burn off the excess fuel and running hot for too long will damage it.

 

Thanks, very helpful. So I will now try replacing the front O2 sensor.
In looking at diagrams and available parts, it appears that there's two O2 sensors in addition to the rear one I replaced- one in the rear exhaust manifold firewall side and one in radiator side. Any idea which one is causing it? Should I just do both?

 

Bank 1 is the group of cylinders closest to the firewall of the engine compartment, i.e.furthest from the rad, and the problem is with the sensor you described as in the rear exhaust manifold firewall side. Check the connector and wiring very carefully because this one is most prone to wiring shorts due to insulation damaged by heat.
At 62K I don't think you need worry about the other sensors, but if you want to be fussy, and provided all sensors are identical, putting the oldest sensor furthest downstream in the exhaust system makes a bit of sense. This is because as they age, O2 sensors get a bit lazy, so if the laziest one is placed in the farthest downstream position, it, being the "referee" which makes the call on whether or not to put the CEL on, will be more lenient in making the call-- but it's really no big deal.

(The nice thing about six cylinder engines as opposed to fours like mine, is that with a six, when one cylinder craps out on you, you've got a spare).