Check Engine Light plays tag with Gas Gauge

You need to have a second opinion. Take your car to an other shop - preferably to a Subaru specialty shop and ask them to diagnose it again. There might be a cat. efficiency code in the ECM, but thee should be an EVAP problem also. The ECM won't run the EVAP leak test if you have more than 80 and less than 20% fuel in the tank. It seems there is an EVAP problem which could be as simple as a new gas cap or a vacuum hose or it could be something which requires more diagnostics. Zee

The fact that your CAT code comes and goes is common when the CAT is in the early stages of failure, but eventually it will stay lit all the time. The CAT Monitor, which sets the code, runs most frequently when you are on the freeway. Keep this is mind. good Luck As for the 20% - 80% fuel level, most of the Monitors will not run, including the CAT Monitor on some models, but especially the EVAP Monitor. As for a Subaru shop, I would look for Emission Experts, I do Emissions work in California and I repair cars for several Dealerships on a weekly basis. Emission Diagnosis has become its own specialty. Subarus have their own pattern failures and worn CATS are very common as are leaking gas caps. The rest of the EVAP system on a Subarus, generally, tend to be pretty bullet proof. Most common CAT code is P0420 and leak codes are P0442, small leak, P0440 large leak and P0455 gross leak, all can be caused by a defective cap. Test the cap or get a new one from the Dealer, not an after market cap and then go from there. And yes, keep your fuel level between 1/3 2/3 full until you get through this, that way all the Monitors will run all the time, so you will know where you stand.