I have a 2003 Subaru Legacy L with the EJ251. I rebuilt the engine because of a thrown rod. There was significant damage to the shortblock, which resulted in a significant amount of oil escaping the tailpipe. I replaced the shortblock and rebuilt the engine. After the first ~1hr of engine time there was no oil visibly escaping the tailpipe. I spent about 500 miles resolving a transmission issue and test driving, and just took the car for the first few highway drive tests. After a recent highway drive I got the following codes:

These both point to problems in the exhaust land, but I am not sure how to interpret these into a fix. I think the oil damaged the sensor(s) and cat. I don't know much about exhaust. What is the best way to address these codes? (I live in a state without emissions inspections.)


1 Answer 1


DTC P0420 - Catalyst system efficiency below threshold (bank-1)

DTC P0457 - Evaporative emission control system leak detected (fuel cap loose/off)

First let's look at the P0420 causes include:

  • Exhaust leak
  • Catalytic converter
  • Rear O2 sensor circuit

The computer compares the pre- and post-cat O2 sensors readings to determine if the cat is working effectively.

In the absence of an O2 sensor code, it's likely an exhaust leak or a damaged converter. I would check for an exhaust leak. If that isn't the problem I would put about 3000 miles on it to see if the converter will clear up before condemning the converter.

Now let's look at the P0457, causes include:

  • Loose or missing gas cap
  • Vacuum lines cracked missing or misplaced
  • Fuel filler pipe packing
  • Drain valve
  • Purge control solenoid valve
  • Pressure control solenoid
  • Canister
  • Fuel tank

The computer pulls a vacuum on the fuel tank and measures the leak rate. If this rate is too high, it sets the code. Any leak in the fuel system on the vapor side will set this code.

Considering you just put a short block in, I would concentrate on finding a vacuum leak. A misplaced or disconnected line. Vacuum lines become brittle with age and simply moving them around during the engine rebuild may have caused one to crack.

  • Thank you for the great answer! I will chase these down. The short block did get rebuilt by me with a machined crank and rods, I didn't just "drop it in" from a crate. I also completely disassembled and cleaned the heads. Could mistakes in any of this introduce an internal vacuum leak? Dec 28, 2014 at 20:47
  • 1
    @AlexHirzel It's not likely anything internal. Dec 28, 2014 at 21:41

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