If anyone has any insight, or, I'll just document what I find in case someone is looking in the future.

 

My mom's 2012 Impreza 2.0i Premium. 216k miles, original engine and CVT. Had the torque converter solenoid fail last summer, and the fluid was a bit brown, so I did a drain and fill then (probably 190k miles), and we did another at about 80k, when I was working at a dealership.

The AT Oil Temp light was flashing one day, I scanned it about 2 days later (no lights on) and it had a stored P0841, as well as a U0100 (CAN signal error), C1431 (AT abnormal), and C1422 (VDC Interrupted).

From the Subaru FSM:

Quote

J: DTC P0841 SECONDARY OIL PRESSURE SENSOR PERFORMANCE

DTC DETECTING CONDITION: • Control valve hydraulic circuit malfunction • Secondary pressure sensor fault or characteristics malfunction

TROUBLE SYMPTOM: • Acceleration is poor during standing start. • Shift control malfunction • Engine speed increases abruptly.

First 2 steps in the diagnostic procedure is to check the condition and level of the CVT fluid. I think she's going to have another drain and fill done (she does not live locally to me, so I probably won't see the car for another couple weeks). And then there are a few tests using the SSM to check the pressure readings from the sensor under a few conditions (Idle in park, 3k rpm in park, torque convert stall at WOT). 

 

Beyond a fluid related issue, the procedure basically ends with the sensor, wiring, or transmission causing the fault.

 

The secondary pressure sensor is externally mounted, threaded in towards the rear of the left side of the transmission. By her VIN, part #31878AA020, with an MSRP of $148.17

Edited September 14, 20223 yr by Numbchux

co-worker had something like this.  He got the sensor from the yard and put it in.  problem solved.

Well, small update.

 

She had the fluid changed at a local-to-her independent shop that she frequents, she brought in Subaru CVTF

A few days later, she was driving, and noticed a loss of power on the highway, and when she pulled over, the engine had stalled. She had to navigate across a lane of traffic and an entrance ramp, so she's not sure if it stalled on the move, or when she stopped (like a stuck torque converter). But, after cycling the key, it started back up, and drove normally. My dad read the codes later that day, and found P0700 (AT abnormal) and P0841 again.

 

She's visiting me again this week. First thing I found out, none of my scan tools will read the live data for that sensor. I have a Carista ELM327 bluetooth adapter that I use with my phone, I have a couple basic apps, but ActiveOBD is a Subaru specific one, with which I can read torque converter lockup, AWD transfer, transmission temp and more. I also have an Innova 3170RS, and FreeSSM.

 

So, then I was left with the options to buy a better tool to test it. Pay the dealership to test it. Or, just throw the $150 sensor in. The local dealership had it on the shelf, and said they use them in the shop about once a month. If it were my car, and only driven locally, I'd drive it and wait for symptoms to worsen. But it's my mom's, and she drives all over the state, a breakdown in the middle of nowhere would be far more costly.

So, I did what I hate to do, and loaded up the parts cannon. Yesterday I put 5 more quarts of Subaru CVT fluid in (what came out was still pretty brown) and the secondary pressure sensor. With such intermittent symptoms, only time will tell if that was the chicken or the egg. I drove it to work today, and it's showing it's age.

 

Not sure what the future is for this car, it seems like a pretty nice car, but with a salvage title, considerable oil consumption, and a failing CVT, it's probably not worth much, and not worth fixing.

We stuck the fork in it. The codes haven't come back, but it slips and lurches from a stop (especially uphill), and makes an awful noise.

 

We just put a deposit on a 2018 Impreza hatch with 17k miles and a salvage title due to hail damage. I tried to get her out of a Subaru, but a comparable Toyota (Rav4, AWD Camry, Corolla Cross, etc.) is at least $10k more.