Diag code (P0A84) without check engine light

Sounds non trivial.
Maybe these might give more insight?
P0A84 Hybrid Battery Pack Cooling Fan 1 Control Circuit Low
P0A84 TOYOTA Hybrid Battery Pack Cooling Fan 1 Control Circuit Low

 

is it under warranty?

 

2 month old 2018? I would hope so.

 

Toyota’s Repair Manual for the Prius Prime (more info) lists several diagnostic trouble codes (DTCs) for battery cooling fans (including P0A8111, P0A8115, P0A8196, P0A9611, P0A9615, and P0A9696), but it also says that if any of these is present, the master warning light (triangle with exclamation point, ⚠️) should also come on.

If the master warning light on your car is lit briefly during the lamp test when the car is started, but doesn’t come on or stay on after that, then I’d be inclined to suspect an anomalous reading from the third-party diagnostic tool. I’d suggest not clearing the codes again, however, to avoid erasing information that might be needed for diagnosis, and waiting for the warning light or other signs of trouble.

If the master warning light is on, you should ask your dealer to investigate, of course. They’ll use a Toyota Techstream diagnostic system, which can read the six-digit DTCs used on the Prius Prime.

 

Is it a pending code? Some codes need to exhibit for a bit before they actually trigger the triangle and move into true trouble code category.

 

Carista just published Ver 3.7.3 of the app. Addresses Toyota/Lexus Diagnostics for 2017+ the changlog says.

 

That’s a good question, but to answer it, we’d need to know how your car responds to a scan tool that uses only the capabilities that are required to be in the car by the Federal and California regulations relating to OBD II and the industry standards, such as Society of Automotive Engineers (SAE) J1978, SAE J1979, and SAE J2012, incorporated into those regulations.

The diagnostic trouble code (DTC) itself, “P0A84,” is defined in SAE J2012-DA, the digital annex to SAE Surface Vehicle Recommended Practice J2012 DEC2016, “Diagnostic Trouble Code Definitions,” which says it means “Hybrid/EV Battery Pack Cooling Fan 1 Control Circuit Low.” That standard is really just a big spreadsheet listing of codes and their meanings, though.

The more important question is how the scan tools are finding the DTC. If they are pretending to be Techstream, for example—interacting with the car using automaker-specific protocols or techniques that were found by reverse engineering, rather than by following only the interfaces defined in the OBD II regulations and standards—then the scan tools may simply be wrong.

If the scan tools are just asking your car for confirmed, pending, or permanent emissions-related DTCs according to the OBD II standards (i.e., Services $03, $07, and $0A from SAE J1979), and it’s reporting the P0A84 code, then it’s likely that there is or was an actual malfunction (even if Techstream isn’t finding the DTC, for whatever reason), or there is a deficiency in the car’s OBD II implementation, for which Toyota would, in principle, need to provide a software fix or seek a waiver from the authorities.

It may be tricky to get a dealer to spend much more time on this: if the battery fan works and the circuit is OK, Techstream isn’t finding any DTCs, there are no warning lights or other symptoms, and you don’t have a report of a failed emissions inspection in hand, you’ve reached the limits of their diagnostic and repair capability.

I don’t think it’s worth pursuing, unless you have the skills and equipment to investigate the matter yourself, but if you had a bit-by-bit trace of a scan tool’s communication with the car, I suppose you could desk-check it against the OBD II standards. If the scan tool isn‘t doing anything strange, the next step would be to ask the dealer to open a case with Toyota’s Technical Assistance System (TAS), the dealer-only support line, and submit your findings. Hopefully Toyota would address the matter without the need to involve EPA or CARB.