2009 Subaru Legacy EJ253 Non Turbo 4EAT transmission

The car seems to be in LimpHomeMode although I don't know how to verify that. Very poor acceleration with some hesitation when in drive. The transmission has a manual mode which seems to shift through all gears fine. Shifting 1 through 4 I can only get the car to 45mph with flooring the gas. This car is full drive by wire but I'm not getting any DTC codes.

before this low power issue was encountered there was work done on the engine: Timing belt broke, bent valves, engine was removed, cylinder heads removed and new valves were reseated into cylinder heads from a junkyard. Cylinder heads were machined before reseated and gasketed back to the engine.

The car drove for months after this valve job with good performance. There was a complaint of hard shifting between 2nd-3rd gear, slight shudder when accelerating. At this time the transmission pan was dropped for inspection and the pan cleaned and new fluid added plus 16oz of transx fluid. No abnormality's or bits or metal shavings in the pan.

After the transx treatment the car drove without the shudder for weeks.

Here's some things I have suspected and either ruled out or replaced

Transmission fluid had small bubbles apparent on the dipstick after a hard test drive: flushed and filled trans fluid (removed the trans coolant line and cycled through 12 qts new fluid). This step removed the transx fluid. castrol transmax transmission fluid added

Cleaned MAF sensor to no effect

Dirty Fuel filter: Engine runs great at idle, seems like there should be a DTC leading to a potential bad fuel filter

Throttle position sensor: How do you test this?

Bad trans shift solenoid: The car shifts 1 though 4 fine when in manual mode.

A Vacuum leak: Thorough visual inspection found no leaks, seems like this would lead to a DTC

Tested Camshaft Position Sensors: tested good. (these sensors came along with the heads from the junkyard)

Removed positive battery terminal for 15 mins and test drove: same issues with poor acceleration, no change

Since there are no DTC's I'm stuck with what to test next?

  • The next thing I'd do is read live data directly from the ECU. This may give you indications as to the fueling or knock or other things which might be going on, yet wouldn't be showing through a DTC code. Commented Apr 18, 2021 at 15:31
  • I have swapped the MAF sensor out with a working donor vehicle sensor. No change in condition with poor performance poor acceleration.
    – D A
    Commented Apr 18, 2021 at 17:59
  • I have the FOXWELL obd2 tool and it can read live data but its a clunky interface, can you point me towards what I should be looking for?
    – D A
    Commented Apr 18, 2021 at 18:00
  • It should allow you to see what the throttle position is at all times. You can check your O2 sensors to see what they'll tell you. Look at everything and see what you can see. If something looks out of place, post it up. Commented Apr 19, 2021 at 1:02

1 Answer 1


I've seen several EJ253's of this generation suffer failed catalytic converters where the catalytic substrate has physically broken up in the exhaust meaning the car cannot breathe properly with no DTC's. A car we worked on could barely make it up a local hill but after replacing the cat it was no problem.

Also I guess you've checked the battery voltage is good and with the grounds all good, were all of the ground points reconnected after the work described.

Of course your car could have something else wrong but you can drop the front exhaust to inspect. If you shake the removed part it should not rattle or have pieces of the catalytic substrate fall out! If it does that will need replacing.

RomRaider is a good & free software to log Subaru data, you need to buy a VAG-COM cable to connect a laptop https://www.romraider.com/. Log MAF g/s, load, RPM, AF, O2 values under hard acceleration whilst in Drive, feel free to post the results back here. Please let us know how it works out or if you found the fix.

  • 1
    You nailed it, I took the car into my fav mechanic this past week and he diagnosed it as a clogged muffler. I removed the first o2 sensor at his shop to allow for the pressure to release and then drove it to the muffler shop with a noticeable improvement in power.
    – D A
    Commented May 15, 2021 at 9:53

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