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Engine Light on after 50+ mph - Subaru

I have a 2005 Subaru Legacy. The engine light comes on whenever I go over 50 mph. However, after the car has been restarted 3-4 times for city driving, it goes off and stays off until I get on the highway again.



When it first came on, I took it in and the code read P0131 (O2 sensor). However, the mechanic did not think it was necessary to replace the sensor, took the code off, and said to come back if the light came on again. I haven’t taken it back yet.



Can someone tell me why the engine light goes off and on? Is this a major problem that needs to be fixed immediately or can it wait?

Well I would start by saying that code means the computer is getting some indication that the values coming from that sensor are not what they should be. It may be the sensor but remember it may be what the sensor is sensing and not the sensor.

Someone who knows that car better than I do (which is a lot of people) might have a good idea.  Your mechanics suggestion was not unreasonable.

That code indicates low voltage to the upstream (before the catalytic converter) O2 sensor - not a problem with the sensor itself (necessarily).

So it means that the car’s computer detected low voltage - this can be a problem in the wiring to the sensor or a problem with the sensor itself. O2 sensor’s are not all that cheap and you would want to ask the shop to check the circuit before just replacing the sensor.

It is not an emergency - so it can wait but I would never let anything like that go a long time.

If the engine light ever starts to flash - that’s an emergency - don’t drive the car until the problem is sorted out.

The Check Engine Light (CEL) is an early warning system. It’s job is to alert you to a developing problem BEFORE it becomes a bigger, more expensive-to-repair problem. You ignore the warning at your economic peril.

The output from the O2 sensor can be measured and tested. If it’s going out of range it could be the problem. To GUESS that the O2 sensor is not the problem is not a good thing.

The fact that the light goes off after a few start cycles indicates that something is sending a signal that is within normal range most of the time, but goes out of range now and then. What makes 50 mph the magic number? Wish I knew.

The light could be indicating the O2 sensor again, or it could have a different code this time. The only way to know is to have it read.

If this were my car I would probably wait until the light stays on solidly for several days, and then take the car to a mechanic.

You can wait a LITTLE while, but you shouldn’t ignore the light once it comes on and stays on.

I would visit the dealer if under 8yr/80k. There are a rash of cat converters dying on 05, 06 Subaru with non turbo engines.

Thanks to you all for your information! I actually had the dealership check the codes first since I was there for other things and they said nothing other than how much it would cost to replace the sensor. However, I don’t fully trust them, which is why I got the second opinion at another local shop. It was there that they told me to wait and see what happens. Nevertheless, I will get it checked out. Thanks!

It makes a difference in how the DTC (Diagnostic Trouble Code) is analysed, whether the engine is a 4 cylinder, or a 6 cylinder. Which is it?
A 4 cylinder engine will have only one forward oxygen (O2) sensor; whereas, a V6 will have one forward O2 sensor per bank. There are two banks of cylinders on a V engine.
Subaru Legacys have 4 cylinder, and V6 cylinder engines.
What’s all this precision, that I speak of, have to do with anything, anyway?
The precision of diagnosis can make the difference between whether you get charged for a wire repair, or a new oxygen sensor.
The DTC P0131 refers to low voltage to, or from, the front oxygen sensor, on the left bank (Bank 1) of a V6 engine [There are 2 banks], or, just the one front oxygen sensor on the 4 cylinder engine [on which, there is only one front, and one rear O2 sensor] per its one bank.
For your problem (P0131) voltage in the circuit to the O2 sensor should be checked, and the wiring condition, and connections, should be checked. The repair could be something as simple as disconnecting, and reconnecting, wire connectors to the O2 sensor; or, the defect could be elsewhere.
The repair shop may want to try the experimental procedure of: “change the O2 sensor, and see what happens.” (Actually, that is what I expect them to do because it may seem expedient (and profitable), for them. They won’t, actually, admit to that).

Thanks for your help. It’s a 4 cylinder - rear O2 sensor.

DTC P0131 is for “Bank 1, Sensor 1” which is the front oxygen sensor.

Interesting, thanks for the clarification. They told me it was the rear sensor.

I actually had the dealership check the codes first since I was there for other things and they said nothing other than how much it would cost to replace the sensor. However, I don’t fully trust them

Yea, I would not trust them. Unless I am missing something this should be fully covered under the emissions warranty.

Subaru’s use a 4 cylinder horizontally opposed engine. They call it a ‘boxer’ engine. There are two banks, so the same rules apply as a V engine. 4 oxygen sensors.