Subaru 2004 Outback check engine light stays on

Ok you guys once and for all, why does my engine light keep coming on? I replaced my gas cap with an official dealership one. I’ve heard it can be the emissions sensor. I’ve heard that to run it with the engine light on it is no problem. Is it common with this year and model. I’ve just been running it with the light on. My Subaru dealership said that’s ok but it drives me nuts because what if other problems happen and my engine needs to turn the light on? In a quandary. My car runs great. Takes in hills like the wind without shifting down. It has 185000 miles on it. Ok card talk whiz buddies, help.

Unless you live in California, I’d suggest that you go to a nearby auto parts store like AutoZone and have them read to code for free. If you chose AutoZone, they will enter the code from their code reader into their computer and give you a readout of the code and what it means and possible suggestions for repair.

The code will be at the top of the cash register type receipt and post that code here. It will be a letter followed by 4 numbers. For example, it might read P0420 which would be the catalytic converter is below efficiency.

After you get the code, don’t do anything until you post it here. Sometimes there is more to the story than the code might indicate.


… or perhaps as many as a couple of hundred other malfunctioning parts.
As was said, you need to have the stored trouble code(s) read in order to know exactly what the problem(s) is/are. If the CEL has been lit-up for an extended period of time, it is entirely possible that there will be several trouble codes, as these things can… cascade… if the first problem wasn’t rectified promptly.


As long as the performance is ok, there’s usually not much problem as far as the car goes to drive with the CEL on (but not flashing) for a while at least. You’ve already ID’d the issue where you don’t know if another problem has developed or not. So you are right, it is a good idea to get to the bottom of why that pesky light is on. Anytime the CEL turns on at least one diagnostic code is stored, so as mentioned above by my esteemed Car Talk colleagues, that’s where to start. Doing anything else would just be random guesses. It is easy to run out of money before running out of guesses.

Read the codes, even CA allows auto stores like O’Reillys and Autozone to do it now. Or just buy a reader. Generally the a steady light just means either some emissions equipment has failed or that the engine can’t stay within emissions spec. This often can be tolerated for awhile but also could put the car into a “safe” mode such as running richer to avoid running hot and burning a valve or piston, but which also can age your carylitic converter more rapidly, etc., it depends on what is wrong. Determine whether your light will switch to blinking if a problem that threatens immediate damage occurs, if so you have some reassurance but if not you’re running blind with it on. If your state requires inspections you’ll need to fix it to pass so may as well do it soon. Also check the Subaru and outback forums, like and look up your model.

I’m going to have the light looked at. Had code read 3 times and it just said gas cap. We get a lot of rain in Oregon. Thought it might be moisture tripping it as it happened a couple of times in pouring rain storms. I went to dealership. They said not to worry about it but running it blind with the light on bugs me. It runs great by the way! Don’t know. I will get back to you. It’s a mystery. I’ve driven it since about 9000 miles. I’m bad but I’m poor. The car has 189000 miles on it.

There is not a code that says gas cap. As keith pointed out, the codes will be a letter followed by 4 numbers. Post those codes here - not someone’s hunch. Then it can be discussed.

Your Outback has an “evap system” which prevents gasoline fumes from leaking outside the car and into the atmosphere. When you are driving the car it’s a sealed system, and the engine computer occasionally tests how well sealed it is. Apparently yours has a leak. There’s a half dozen ways it can leak, the gas cap is just one of them. One of the most common culprits that causes evap system problems is the purge valve. That is what is used to allow the engine to burn off the fumes that get stored in the charcoal canister during refueling.

The evap leak code merely means the system wasn’t able to generate a sufficient vacuum within the specified time frame

We know it’s not the cap . . .

That leaves fuel filler hose/neck, fuel tank, fuel sender seal(s), evap purge valve, evap vent valve, and so on

See where this is headed?

My advice is to take the car to a reputable shop and ask them to diagnose the car and provide a repair estimate


Thanks. I have had it read before there. I will get it done again. I have a great auto repair shop that did my head gaskets and has turned off the light but I will need it done again. I read a couple of great articles about the Subaru light coming on and all the reasons why it might. It is All Wheel Drive Auto (AWD) and was very complete. I understand it can be sensors for the converter coming on and because I had a bit of time with head gaskets Bad and leaky antifreeze, it could of messed up my converter. Great information there. I will get it looked at and analyzed. They say good idea having someone do it who is trained in admissions.

Or it could be like you say too with evap leak code. I do smell some gas fumes when the car goes down the road when it’s full and windows are open. I will get codes read again. Taking your suggestion to check this out, evap leak.

Thanks I will get the code read again. I am in Oregon and so no emissions testing on my car.

Ok thanks. I knew the code said more things than gas cap but I did go there first and got an official Subaru one.

Yes random guesses but good information to learn and I love to learn about cars. I’ll start with my good mechanic where I had my code read before and light off but it will need to put on another round of turning on again to get code perhaps. They are used to Subaru there and very good mechanics. It will be interesting. I will mention the gas smell too as possible leak. Hopefully not the converter. My car runs great after 189000 miles on it. Hope to get another 100000 on it. Has head gaskets newly replaced.

Ok everyone thanks for all your suggestions. I will see who wins? At least as a woman I won’t go in there being stupid or perhaps my being overinformed will just cause me to look screwy. My mechanic starts from ground 0 and gets to the bottom of the problem. They see a lot of Subarus. At least I learned some things online today and here
Old Laughing lady, me,
Driving my lemon, but it’s doing well lately! It has hail damage on it but is good for off road.

Ok I took my car into a reputable repair guy, Gary and K, who have been good to my former employer servicing all their cars. I talked about evap test and they did a smoke test on it and found broken connection onto a part near the manifold. They said it probably runs rich. The guy couldn’t understand why the light did not come back on but I’m waiting for it. They found a suspension problem so fixed that and I will get the evap problem fixed next month. Thanks for all your help. I went in there and actually had car conversations! I love to watch repairs. They treated me well as a woman and showed me everything. Thank you car talk people. I will bring my car back there. You all were right. Evap system leak. Smoke test done.

Also a good blog by seattle FWD Subaru gave a complete article on all the problems that can come up when the engine light comes on. Prevents a person from yelling at the mechanic when they don’t get it right because it can be a number of things wrong! Get informed before you get frustrated and rip into your clueless echanic.

Glad you got it solved. Happy motoring :slight_smile:

Awell now an even bigger problem! Leaking transmission fluid. They checked. I need a new driveshaft and some parts seals so will have to park it again to save up money. This lemon never ceases to give me lemonade. Still I love it, my project car. Glad I’m retired so I can keep adding to it. Going to fix door that stays locked in the back, remove some dents shine it up while it stays parked for 3 months. Thanks for reply. Stay tuned to my Subaru saga.

I will, but you have to realize that a 15 year old car with almost 200k miles is nothing more and nothing less than an old car. That is not at all what constitutes a “lemon”.

If you doubt me, please take a look at the text of your state’s Lemon Law. While these statutes vary to some extent from one state to another, NONE of them cover 15 year old cars with almost 200k miles.
Ergo… your car is not a “lemon”, and is just an old car that is in need of repairs that will only increase as the mileage on the odometer increases.