P0133 code on 1999 Forester

[tbolt1003]

I've searched through the forums here and think I have a general idea of where to look.

 

We just had the head gaskets replaced on our 1999 Forester with 125,000 miles. Car was going through coolant and oil at a rediculous pace. Prior to the repair, there were no trouble codes. We got the car back this past Thursday and it runs great. Last night, however, the CEL light came on and when I scanned it, I got P0133, O2 sensor circuit, Slow Response Bank 1, Sensor 1. The car is now idling rough but didn't prior to the code appearing. 

 

I've found out this O2 sensor is on the right side of the car prior to the cat. Could the excessive coolant and oil usage caused the sensor to act up and cause this code to appear? I haven't had a chance to check the wiring or the connector yet, perhaps the shop forgot to plug the sensor back in. I can't hear any exhaust leaks. Am I on the right track here? Anything else I should look for/at? Thanks!

 

[Fairtax4me]

Coolant in the exhaust can foul the sensor and cause this code.

You may want to check the wiring and connector, but most likely will need to replace the sensor.

[tbolt1003]

Thanks for the response, Fairtax. Knew you'd be one of the ones to respond. All the electrical connections look good, so going with it being a bad sensor. The engine is idling rough at a stop and in gear now.

 

I've read negative comments about going with an aftermarket sensor and many suggest getting an OEM unit right from the dealership. Any truth to this? If not, any particular brand I should consider and/or stay away from? Thanks!

[Fairtax4me]

It's usually best to match the brand of the current sensor that's on the car. If you look at the sensor body you can often see the brand name stamped or engraved in the case. If you can't find the brand on the sensor, go with Denso. Japanese car, Japanese parts, they'll play well together.

 

Also be sure to check the shape of the wire connector for the sensor. There are tabs on the connector that prevent an incorrect sensor from being plugged in. There may be listings for several different O2 sensors for the same car, and the only way to tell which is the correct one is to look at the connector.

[Olnick]

Or . . . an alternate point of view!  Last O2 sensor I put in was a Bosch Universal--purchased at a very good price off Amazon.  Car ran beautifully for the rest of its life.

[Fairtax4me]

Or . . . an alternate point of view! Last O2 sensor I put in was a Bosch Universal--purchased at a very good price off Amazon. Car ran beautifully for the rest of its life.

This I don't recommend on many newer cars. There are big differences in the way the modern wide-band sensors work compared to the older standard oxygen sensors. This also applies to most California emmissions spec vehicles.

The newer wide-band sensors use a bias voltage from the ECU which makes the sensor output more sensitive.

 

The universal sensors are standard oxygen sensors not designed to use a bias. They create their own signal voltage, and will not respond properly or can be damaged if used in a car that sends a bias voltage to the sensor.

 

They work great and are a cost saving alternative if you know your car has that type of sensor. But will bite you if your car needs a biased sensor.

 

This is the reason for closely checking the tabs on the connector. If you can determine that the sensor on your car is a standard O2 sensor, a universal will probably work, you just have to so some cutting and twisting of wires.

 

Sometimes its worth the extra money to not have to fiddle with wires and to know you have the correct sensor.

[Olnick]

Fascinating stuff, Fairtax.  So when did they change to the "modern" sensors?  (Shows how out-of-date I am--I've never even touched a Subaru newer than '96!)

[Fairtax4me]

They've been in use in some cars since the early 90's, but weren't very common. Subaru started using them as early as 97 from what I can tell, though again, not very common. Late 90's and early 2000's they became much more popular. Anything made in the mid to late 2000's almost certainly has wide-band sensors for the front sensors.

When Subaru went to the 5 sensor setup on the 2.5 engines in 05, they used wide-band front sensors. As far as I know all Subaru models after that also use wide-band sensors for at least the front sensors.

They're kind of mix 'n match in the years before that. Many manufacturers were working on fine-tuning new emissions control systems in order to keep up with changing regulations. Two same model and year vehicles could have two entirely different emissions systems, yet outwardly appear no different.

 

Most of the time in the late 90's to early 2000's if a car had wide-band sensors it would be labeled as having CA emissions specs, but not always. And this is where carefully checking the sensor connector becomes important.

[tbolt1003]

Thanks for the replies. The sensor looks to be the original one. I see what you mean, Fairtax, about the different style connectors and making sure I get the right one. Denso has the sensor with the correct plug, so I'm going to go with that one. A little less expensive than an OE sensor, too.

[tbolt1003]

Also, we are now getting another code for a Misfire on Cylinder 4. I had just replaced the plugs and wires with NGK's a mere month before the head gaskets blew. Good chance the plugs are needing replaced, as well, due to the coolant in the combustion chamber?