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2000 Subaru Forester - CEL P0133, Oxygen sensor

Hi all,

My 2000 Forester has 96,000 miles on it. A few months ago, the CEL came on. At first it would turn off every couple of days but would eventually pop back on. I first thought it was tied to putting gas in the car, now I think it was just a coincidence.

Over the past two months, I have replaced the catalytic converter with a after market piece (done by a mom and pop muffler shop) and replaced the front o2 sensor three times. The first two times, I used Bosch sensors from an auto parts store and had the code cleared. After about 50 miles the CEL came back on. Just yesterday, I picked my car up from the dealership where they installed a Subaru fron o2. After about 30 miles, the light is back on again. Code is P0133 - heated oxygen sensor, bank 1 sensor 1, circuit slow response.

Now, the dealership is suggesting I need to replace the catalytic converter with a Subaru converter before they can source the issue.

Does anyone have insight into what the issue might me? A less expensive approach to sourcing the problem? Repair ideas? After whats been done, I still don’t feel any closer to getting to the bottom of this!


The first piece of insight you need is that you need to take the car elsewhere. Someone at that dealership has their head up their keister. Was the converter originally replaced because of this code??? Either way the fact that they are now suggesting it is crazy. That code has nothing to do with the converter. It is an error code regarding the upstream sensor - it is before the converter (sensor 1). Nothing about its response has anything to do with the converter.

And…now that the sensor has been replaced 4 times (or did I lose count?) I’d bet its not the sensor itself. So take it to a shop that knows a) what an “upstream” sensor is; b) what a “circuit” is. Note the code - it doesn’t refer to the sensor itself. It refers to its *circuit." Its time for someone to realize that the sensor circuit involves the sensor, its wiring, and the PCM.

Unless passing an emissions test is an issue, spending a lot of money on this makes little sense…This is NOT a life-threatening situation.Spending hundreds if not thousands of dollars to make that little light go off is insane…If your car is operating properly then drive on…

Sta away from monstersalvage. They want to chage u to inquire about parts. Looks crooked and sleazy.

Cigroller - that was my gut reaction too, that they were milking me dry. I’ll find another shop to pursue this :slight_smile:

Caddyman - I’m with you, that putting more money into this, what appears to be a minor/non existent issue. seems wasteful. However, my main concern is that if I can’t get the cel off, I won’t know when a bigger issue pops up. Not to mention the annoyance factor of seeing that light every time I drive…

Kriley - thanks for the heads up!

I would do an exhaust leak test. This should not be a difficult problem to find and correct. (If it were a front O2 sensor problem, it would not require 50 miles to regenerate a DTC, more likely only 5-10 miles.)

Here is a link you can refer to for this trouble. Checking for an exhaust leak would be good thing to do alright.

“my main concern is that if I can’t get the cel off, I won’t know when a bigger issue pops up. Not to mention the annoyance factor of seeing that light every time I drive…”

Urban legend…Cars operated for 80 years without any CEL…If a 'bigger issue" pops up, you will surely know about it… If the light bothers you, a little square of black tape takes care of that…Or let the repair shops go on a fishing expedition at your expense…

Urband fact … Cars operated 80 years without fuel injection … If a “bigger issue” pops up, like an overly rich mixture in your catalytic convertor, you will surely know about it.

Do yourself a favor, get it done sooner rather than later.


Took the car to an exhaust shop, and they could find no leak. So, it seems that rules out both the oxygen sensor and leaky exhaust…

The mechanic mentioned that it sounded like the car was misfiring a bit - though I can’t find a connection between this and the P0133 code. Before I take it to the local Subaru shop, any other suggestions?

Thank you all for your insight!

Someone has to check the circuit for that sensor. That means the actual wires, including the wiring harness for the sensor and perhaps the connections at the PCM.

A check for vacuum leaks & cleaning of the MAF sensor would also be good.

If there’s a misfire involved a compression test should be performed; something that should be done with every engine performance complaint even if a misfire does not exist.

There’s always the possibility of valve lash tightening up on one or more valves and if lash is tight on an exhaust valve this usually means engine work will be needed; either immeidately or in the unforeseen future.

Just my humble opinion, but valve lash should be checked at about a 1000 miles and every 30k miles afterwards although Subaru does not recommend this procedure; a procedure that they used to insist on.
The current recommendation of over a 100k miles is flawed from a purely mechanical standpoint and leaving the marketing department clean out of it.

I suggest you have the shop make sure the heater circuit for the O2 sensor is getting full power to it. There may be a bad connection to it and it isn’t heating up like it should be.

I’ll second that suggestion Cougar. The heated O2 sensor may not have full voltage to the heating coil and it may have a poor ground. It’s a shame that no one has apparently taken the time to get down there and get the facts. A 4 channel digital scope might zero in on the problem in a few minutes.

The problem is not in the heater circuit. Exhaust gases raise the temperature to operating level in a few minutes without the heater circuit. The heater simply raises the temp to operating point much sooner.

The problem here is that the O2 sensor output flips too slowly between rich and lean. One does not need a scope to look at the O2 sensor output as seen by the ECU; that can be read directly by a scanner attached to the DCL. Doing it this way also checks the wiring between sensor and ECU.

I’d look at the O2 sensor signals first, then use a smoke device to look for leaks in the exhaust system between manifold and O2 sensor. (One can also use a shop vac and soapy water to look for bubbles at the leak site, but I’ve never tried that.)

“O2 sensor output as seen by the ECU; that can be read directly by a scanner attached to the DCL. Doing it this way also checks the wiring between sensor and ECU.”

While a perfectly great thing to do, nothing about putting a scanner on it checks the wiring between sensor & ECU - unless that’s just to say you’ll be able to tell if there’s no connection at all. It just tells you what the ECU sees - not what its supposed to see given the sensor output. If the wiring is good, the ECU will see the actual sensor output. But the wiring can have multiple kinds of problems, not all of which will fall under the category of “not connected at all.”

Another thought, check for a intake air leak on that side of the engine at the intake manifold, after the MAF sensor.

Intake leak maybe, more likely a leak on the exhaust side.

At idle the O2 can cool and function poorly. A poor ground will reduce the current at the heating element and feedback from the heating citcuit’s poor ground can also cause an incorrect signal. A 4 channel Tektronic can give a great deal of insight into such problems. It might be worthwhile to tap directly into the O2 pigtail as part of the diagnosis. But yes, a leaky intake is a good possibility.

I agree a exhaust leak is a more likely suspect but apparently that has been ruled out. As far as the heater circuit goes it may help to check the current flow of the heater lead using a amp clamp to see if normal current is flowing through it.