It's probably your battery. How old is it? Is it the original? I've seen similar symptoms with others and it has always turned out to be a weak or failing battery.

Your systems are probably not getting the correct voltage anymore and are blipping out and causing errors.

The one thing in common all those errors have are your electrical system, and at the top of your electrical system is the battery.

One thing we can all agree on is that the OEM battery sucks.

Think about what happens to your home electronics during a brownout. It's similar to what your son's forester is experiencing.

 

Here's where I admit to ignorance in all Subaru tech.
Wouldn't the alternator keep the voltage up at idle RPM and beyond?

 

Communication systems errors. Check your battery. Check your connections. Check your grounds.

The ECM is robust in its stand alone capacity. It's communications are rather more delicate. Whispers.

 

I told him to check running voltage before doing anything else.
That a bad battery could shut down a running engine is a very foreign concept to me.
Isn't the rectifier/regulator built into it, with the output going directly to the battery?

 

Looks like you have a P0700 DTC stored but I don't see any transmission related DTCs. Does your scan tool have the ability to read the TCM?

The kid was slowing down to a stop light when his car started running rough, then went into limp mode.

P2004/2005 are TGV DTCs, this could be related.

 

Looks like you have a P0700 DTC stored but I don't see any transmission related DTCs. Does your scan tool have the ability to read the TCM?



P2004/2005 are TGV DTCs, this could be related.

Have no idea, he's about 2000 miles from me, but I'll check.

 

@Divemaster5734 even if the alternator is in good shape, a battery in poor condition will cause all sorts of electronic gremlins. Lots of these surface at a stop light when the auto start/stop feature activates.

replace his battery with a Group 24F AGM (I got an Odyssey for maximum performance) and the problems will likely go away.

 

You might see benefit by considering the grounding circuits as two different types. One is the power ground that you are familiar with and the other is a reference ground. The reference ground is computer communication type stuff. Consider that ECU voltages of 0.5 or less are present. So those signal paths need to be clean and the connections secure. I monitor voltage at the ECU all the time with an OBD monitor device. Mine is UltraGauge. Here is a page from my manual on the ECU.

 

Huge Thank You to Everyone!
He's taking the grandson to swimming lessons now, so I have to wait to see how the battery tests.
I have to say, the Subaru dedication is impressive.
It's as bad as my gang with our coal burners.

 

If he ends up replacing the battery himself. Show him this. You can damage a sensor on the negative cable.

 

@aleekat Sent him the pdf.
Much Appreciated.

 

Update:
Got a DVM sent out to him and this morning the disconnected battery voltage was 12.4V.
Had him hook it up and started.
Then it read 14.1V @ 11A.
I thought the battery voltage seemed a little low, I usually get high 12's or low 13's in all my trucks and bikes, the alternator output appeared perfect, but the 11 amps at idle seems high to me, as all mine are usually around 5-8 amps @ idle and no accessories.
He did buy the car new, and it is the OEM battery, so I told him it probably needs replacing anyway.
That said, after having both battery leads disconnected overnight the ECU didn't reset as his BT hooked up immediately.
I'm not the kind of guy to just start replacing random stuff hoping to accidentally hit the mark sooner than later, I'm wondering what your thoughts are on the indicated voltages and ampacity.
Thanks

 

the ECU didn't reset as his BT hooked up immediately

The radio has a battery backup for the user settings. This is independent of the entire rest of the vehicle.

Research Subaru smart charging algorithm on Google. There is a bit to learn there. Your answers lie there in the Googleverse for the amperage.

Detailed explanations of the smart charging can also be found here on this website

Will the battery pass a load test? Measuring voltage is one thing. Putting a hard load on it is another.

 

I believe the answer will lie elsewhere. Troubleshooting from a distance without the full story is going to lead you astray. I highly suggest reading the DTCs stored in the TCM before replacing parts.

 

The easiest way to "reset" is to discharge capacitance. The means is to disconnect one battery cable and touch and hold to the other cable. I don't think OBD codes include a "battery code", just a voltage reading. You should get a low or high voltage alarm and some trouble codes from the alternator, I suspect. I don't have the 2019 model.

This is where we started.

My opinion is that they are communication errors.

Anytime you get a code you want to investigate, pull the freeze frame data and see what's up. That would answer some of these electrical questions.

 

The easiest way to "reset" is to discharge capacitance. The means is to disconnect one battery cable and touch and hold to the other cable.

I've been taught disconnect both cables then touch them together?

 

Yeah, that's a safer bet for the most people. Functionally the same.

 

Disconnecting the battery and holding the terminals together didn't reset the ECU.
Pulling the connectors off the ECU didn't do it either.
What's got me is there's error codes from everywhere, and they all went off at the same time.
The battery was old, so we replaced it with a group 35 AGM.
He said it definitely started and ran better with the new battery.
If there were only TCU errors, that would be easy, but aside from the peripheral systems, there's also an internal processing error along with all the communication errors.
The ECU is in the engine compartment, so thankfully easy to access.
The connectors are in good shape, no signs of oxidation, arcing, or pitting on the pins, and the wiring harnesses are still factory routed and secured.
He's tried every reset suggestion ever posted here and on Reddit.
It's crazy that everything went bad just slowing down to a stop light, I would think that with the 12.4VDC at the battery there wouldn't be enough current with the engine running to cause a voltage drop severe enough to create an under voltage condition and FUBAR the ECU.
I'm stumped.
Will call around tomorrow to see if there's a way to remove the ECU and have it tested.

 

I'm stumped

Might consider a more advanced code reader that can read all the computers.

I can recommend the Gearwrench Pro Dongle. Free software upgrades. Bidirectional.

https://a.co/d/hqH0iof

 

How do you know the ECU wasn't reset?

 

Because the codes were never off.
All I have is the ECU in my Ram Laramie Longhorn Ecodiesel, but it's a high end model with all the bells and whistles, there's only the Limited slightly above, they have the same features, mine has leather dash to go with the azz blower seats and western motif.
When an issue came up with the emission system I started popping codes.
Wound up using a reader to reset the codes to prevent limp mode for a couple weeks waiting for a spot at the dealership for the warranty work.
After clearing a code it would always take a few minutes before the computer had an error reading.

 

Not all codes “clear” with a reset. Some require the completion of a drive cycle to clear. Others are simply held in memory even though they don’t trigger a CEL. If you are attempting to drive in a limp mode and the codes are present, that means the problem is still active.

 

Either that or the ECU went FUBAR

 

Maybe. That’s the long way around, not proving it first.

 

Also an expensive guess.

 

What's the best source for online factory repair manuals for these cars?
I think it's a network communication issue with the dynamics control module.
Here's the code order
1) U0122 Stored
2 U0122 Stored
3)U0122 Pending
4)U0122 Pending
5)P2004 Permanent
6)P2005 Permanent
7)U0122 Permanent
8)P060B Permanent
9) U0122 Permanent
I'm guessing the U0122 is the actual fault?
At least that's the most common.
Code #8 is an internal control A/D Processing Performance, and since it's related to the Power system that's probably it.
I'm thinking a decent factory repair manual will let me know exactly which sensor failed that feeds the module that tripped the ECU apocalypse
Anyone know of a decent factory manual with troubleshooting tips online?
Don't mind paying for a subscription or pdf, my main concern is quality.
Haynes are garbage, I found a few online that seem legit, but not sure where to invest.
I have the full three volume 2600 page Ford Truck manual for my MY, and it's the only way I've been able to pull out the diesel to rebuild, full reseal, convert to stick from auto, mod the turbo, put back in, and start it up.
Thanks

 

When I bought my used 98' Forester in '05, I went to buy the full factory OEM printed manual set ( 8 volumes) and it was $600+. I got lucky and found a full printed OEM set on Ebay for about $75. I try and get the OEM manuals for all my vehicles that I get, that I plan on keeping for any length of time. I still have the 98' and it's my daily driver. Love that car, simple to work on and simple to keep. ( I've got a collection of 9 cars and trucks and 12 motorcycles, so I'm familiar with doing my own "wrenching".)
One important thing to check on, with the type of communication error you describe, are the fuses. Believe it or not, you just might have a bad, loose or blown fuse that could be the cause of all your problems. It's the cheapest and the first thing to check. It could also be caused by corrosion within a connector. Sometimes chasing down these types of problems will make you crazy, but don't dismiss the easy things first, you may be surprised!

 

The internet says:

We only advocate OEM manuals on the forum. It's a rule.

https://techinfo.subaru.com/stis/#/online_reference

 

The internet says:

View attachment 607555

We only advocate OEM manuals on the forum. It's a rule.

https://techinfo.subaru.com/stis/#/online_reference

It appears as though that ABS module is a common issue with these, or at least a really good starting point.
Ouch, my ears just popped as I'm getting sucked into this rabbit hole...

 

Caution: Sometimes Google AI doesn't know so it fills in the blanks with what it thinks sounds good.

What you are reading could be a complete guess or simply made up.

 

Caution: Sometimes Google AI doesn't know so it fills in the blanks with what it thinks sounds good.

What you are reading could be a complete guess or simply made up.

True.
They hallucinate often.
Which is why a human is needed to sort out their responses before and action or report can be submitted.
Been taking AI PDU's in spare time.
That said, using the LLM's can really cut through the insane mess that google created with their heinous add generated nightmare.

 

Never thought about using AI.
Brilliant!

 

Finally had to admit defeat, troubleshooting an electrical issue 2000 miles from the car while using a non-technical person's hands was just not feasible.
Had it towed to the dealer.
They replaced a fuse.
All better.
ugh!
In truth, I think what happened was the battery was worn out, it was the original one, six years old.
Sent him a meter to check voltage, after hearing it was sitting at 12.1 volts, had him replace the battery.
Unknown to me was between the time he got the meter and swapped batteries, he went through the fuses, and lost track of which went where.
Then proceeded to just plug fuses into spaces until out of fuses.
Yeah.
I spent more on the AC/DC clamp multimeter and a nice OBDII reader that does real time, transmission and ABS than it cost to tow to the dealer and repair.
AT least we were able to return the reader to Amazon, told him to keep the meter.
That sums up my foray into foreign vehicles, now back to my coal burning 445 cu in diesels.
Thanks for your help everyone.
Forums are without doubt the best way to meet the very best people.

 

lost track of which went where.
Then proceeded to just plug fuses into spaces until out of fuses.
Yeah.

Ouch. Damn.
Boo

I'm glad it was an easy fix once the fuse was found

Not very easy though when you're not there

 

Which fuse was it? Did they say?

 

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