Seems to be a common problem on all models with 5EAT circa 2005 - 2009
I think a lot of people with the affected Subaru models are having the same issue, so hopefully my account can help others avoid costly misdiagnosis and any sleepless nights worrying about a transmission replacement!
This is the only forum that I have found that has correctly diagnosed the problem based on similar conclusions reached by a mechanic in Japan who blogged about his own experience.
My experience:
I have a JDM 2008 Subaru Exiga GT, same 2.0 turbo charged engine and 5EAT as the JDM Legacy GT.
The same transmission is also used on Tribecas and Outbacks as the list below shows.
B9 Tribeca 3.0L 5 AWD 2006-2009 5EAT
Exiga 2.0L 4 FWD/AWD 2008-2011 5EAT
Impreza 2.0/2.5L 5 AWD 2011 5EAT
Legacy GT 2.5 / 3.0L 5 AWD 2005-2011 5EAT
Outback 2.5XT / 3.0R 2.5 / 3.0L 5 AWD 2005-2011 5EAT
Tribeca 3.9/3.6L 5 AWD 2006-2011 5EAT
If you search for 5EAT shift problems by the various models, you will pick up threads in other forums all exhibiting the similar intermittent problem of a hard shift (most commonly the 2nd to 3rd shift) that feels like the car has been hit from the rear.
In my case, the car has 130,000km (80,000miles) and it started exhibiting the hard shift few weeks ago while operating in "i" mode (Eco).
Just completely out-of-the-blue while coasting in traffic at low speed low rpm, "bang!" It felt like the car was rear-ended but the car continued forward, without any problems.
No check engine lights, no AT Oil lights, no nothing and the car continued to operate normally for a week and then the same thing, similar situation - low speed, low rpm another "bang!".
It took me a few times to realise that it was a transmission shift problem and it only occurred in the 2-3 shift under those specific circumstances.
The puzzling thing with this problem was that it could not be replicated as many have experienced. It only occurred intermittently and usually the engine is well and truly warmed-up, it would never happen in the first trip on cold mornings, but the second trip coming back from an outing.
As the car seemed to be operating fine 99% of the time, I had the transmission oil flushed as the transmission fluid was admittedly overdue for a change and it had lost its red tinge.
ATF
Yes - there are plenty of forum discussions about how only the Subaru ATF-HP should be used, and that this is the only way to avoid any shift problems.
For the record, Subaru ATF-HP is manufactured by Idemitsu, and is identical to Idemitsu ATF-HP.
https://www.idemitsulubricants.com/idemitsu/atf/type-hp
And the other issue that seems to comes up is, the difference between Subaru ATF and Subaru ATF-HP. There does appear to have been a time when the these two types were available, but as far as I can tell only the Subaru ATF-HP is available now. Idemitsu site also only shows the ATF-HP.
The Japanese owners manual only specifies Subaru ATF-HP, but interestingly the North American market version of the Legacy owners manual lists several alternatives as below:
Castrol Transmax J
Penzoil ATF-J
https://carmanuals2.com/subaru/legacy-2005-owner-s-manual-88533
You can check the Specifications from the oil company sites, and they will list these oils as being suitable for Subaru ATF. (No mention of "HP" but the specs are similar to the Idemitsu ATF-HP information)
I am currently using Havoline ATF-J after the transmission flush.
https://cglapps.chevron.com/msdspds/PDSDetailPage.aspx?docDataId=260823&docFormat=PDF
Changing the fluid seemed to have fixed the problem, however I was disheartened to find that the problem re-appeared a week later but this time it was a very gentle thump, and not a violent bang as before. Again I was driving in "i" mode.
I could have lived with the gentle thump but back to the drawing board. Because the car seems to be operating fine 99% of the of the time, I found it hard to accept that there was a serious failure in the transmission itself.
I assumed that as there were no error codes, the solenoids, sensors, and switches on the valve body must be all fine, so decided to focus on the transmission fluid/pressures controlling the shift changes.
Remote AT filter
One thing that is common to all of the models with the 5EAT but not mentioned in the owners manuals is the existence of a remote AT filter mounted well away from the transmission itself. I only knew that it was on the car because I had to replace the left headlight bulb recently and had to access that area through the front left tyre well.
If you look up Subaru AT filter, you will find videos on how to replace this remote filter. However, the consensus is that this filter is very unique, and you should only replace it with the genuine Subaru AT filter.
Although it looks like a spin-on engine oil filter, the valve mechanism inside the filter is very different from any engine oil filter.
It is manufactured for Subaru by Tokyo Roki
Oil Filters - tokyo roki co.,ltd. en
In the Japanese owners manual, the service intervals for ATF replacement on models fitted with the 5EAT is only 40,000km(25,000miles)! The interval for the 4 speed version 4EAT is 100,000km like most other Japanese cars.
It is believed this remote AT filter should also be replaced at the same interval, although there is no documentation indicating this.
The lack of awareness on the need to replace the ATF and this remote filter at such short intervals is probably the reason for everyone's woes.
I went to the local Subaru dealer, and paid an extortionist amount for a single genuine AT filter. You can get them on ebay for around US$30 but this was a case of accept no substitute.
Unlike engine oil filters, the Subaru AT filters have a recommended torque setting of 12-16Nm printed on the filter. Sure enough, I found it incredibly hard to remove the filter from the confined space behind the battery on my car, and when I did get it off, it was completely filthy and definitely needed replacement.
With the clean AT filter on, I had my fingers crossed that everything was back to normal but a week later "bang". The violent shift was back.
Most people probably would have given up at this point, and just replaced the valve body but I realised, the shift was gentle before the AT filter change but now back to violent with the new filter....my stupid mistake!
When I pulled the old filter off, I only collected around 200mL (0.2 quart) of dirty oil, so I stupidly only put the same amount back in.
It turns out that when I checked the ATF with the engine warmed up, and running, and just after moving the gear selector through all the positions, the fluid level was very low and I immediately put in 800mL (0.8 quart) to take the fluid to the appropriate level.
Fingers crossed, and this seems to have resolved the problem for now. I am now only driving the car in S or S# mode to hold onto the gears longer and avoid unnecessary low speed upshifts associated with the hard shift timing.
I am hoping that in my case, the Turbine Speed Sensor No.2 is still operating correctly.
The 5EAT Service manual indicates that the AT Oil light on the dashboard will flash in the event of any failure of the solenoids, switches, and sensors on the valve body. The P code mentioned in this thread also narrows that down specifically to the Turbine Speed Sensor No.2 in the valve body.
For reference: Subaru 5EAT Service manual
http://www.voisin.ch/subaru/docs_techniques/2008_impreza_wrx_&_sti_manuel_atelier/mechanism_&_function_(all_2008_models)/automatic_transmission_(from_'08my)_(5at).pdf
For reference: Parts breakdown of 5EAT.
http://www.alltranz.co.nz/catalogues/pdf/523
As it is mentioned in the previous posts, the valve body replacement is unnecessary if you can manage to obtain the Turbine Speed Sensor No.2, however it seems Subaru initially did not offer the sensors, solenoids, and switches separately necessitating the replacement of the entire valve body.
For reference: Labelled photo of the 5EAT valve body.
Note that Turbine Speed Sensor No.1 is located on the outside of the transmission. No.2 is located on the valve body and sits in the ATF.
For reference(in Japanese but with photos):
Japanese mechanic who blogged about his battles with Subaru in Japan to get them to initiate a recall. His car only has 60,000km (37,000miles) on the clock.
https://minkara.carview.co.jp/userid/631409/blog/37139904
He claims that the part number for the valve body has been changed 5 times, suggesting that Subaru is aware of the problem and has been trying to fix the problem. Later versions should be less prone to this problem occurring.
He points to the number "3" stamped on the replacement valve body, compared to the "1" that was removed from his car, and despite the cost of the replacement valve body, the car is driving better than it has ever done before.
He states that Subaru's position in Japan is that there are not enough documented cases to initiate a recall. However, he has heard that they are now offering the Turbine Speed Sensor No.2 as an individual replacement part. (but charging around half the cost of the entire valve body)
If you can source a replacement Turbine Speed Sensor No.2, or if necessary a replacement valve body, the time required to replace it should not be much more than a standard AT transmission fluid service - the valve body and the sensor is immediately accessible by removing the transmission pan.
Lastly:
The Exiga GT's seems especially affected by the problem.
Here's a forum in Singapore (in English) where they parallel import JDM models, and there are many like this in Japanese Forums.
Everyone seems to ultimately end up coughing up for the entire valve body. I guess it's quick and easy for the garage doing the repairs...
gear jerk when shifting up - Subaru Exiga/Exiga GT - Subaru Club SG