Guidance with P0354 and P0A0F, engine shutoff

Looking for guidance on trouble-shooting this issue. This is a Gen 2 (2009) touring model, throwing P0354 (Ignition coil D.primary/secondary - circuit malfunction) and P0A0F (Hybrid drive system - engine failed to start) codes.

For some background, I bought this vehicle in September of 2023 and drove it without issues (after some initial repairs) for over a year without cause for concern. About 6 months ago, it had the infamous inverter pump failure contributing to the unique shutdown process associated with a blown AM2 fuse - acceleration from engine and motor shut down abruptly and most of the multi-information display shut off. The previous owner apparently got tired of replacing the AM2 fuse and replaced it with a 20amp fuse (unbeknownst to me) without identifying the issue or replacing the pump. While searching for the problem, my mechanic (who is largely unfamiliar with hybrid systems) couldn't identify the issue but found a faulty coil and said the spark plugs were long overdue to swap out. He replaced the spark plugs and put in a new coil pack (this was December 2024) [as an aside: I know this mechanic personally and trust him, if he replaced them they didn't have any life left]. This didn't resolve the issue and it shut down on me the same way the morning after I picked it up from him.

Thanks to this forum, I was able to identify what was actually causing the issue and replaced the inverter pump with the help of a friend (also returned AM2 to a 15amp fuse) in February. I didn't realize it was an issue at the time, but the friend added a universal coolant to the well of the pump to prime it and didn't refill the coolant reservoir (it was low but not empty). I ran it for about four days before it first threw the combination of codes noted above (P0354 and P0A0F) and switched entirely over to hybrid power (the engine shut off and wouldn't turn back on). Immediately after that I drained the inverter coolant reservoir and filled it with the recommended TSL coolant, bleeding the line in the process. This didn't seem to have any impact on the issue as it threw the same codes and the engine shut down again about three days later.

The first time it happened, I turned it off and turned it back on and the engine started right up. It's done it a few times since then and I had to clear the codes to get the engine to start up again, but after clearing the codes it runs like it has no issues for a few days or more before happening again. One of the times it happened it also threw P0420 (Catalytic converter system, bank 1 - efficiency below threshold) and P2238 (Heated oxygen sensor (H025) 1, bank 1, positive current control - circuit low) codes - though these were listed as Pending while the primary two are listed as Confirmed.

Anecdotal, but it typically seems to happen in low gear under strain (going from stopped to moving and going slowly up a steep hill) and seems to happen more frequently when the environmental temperatures are hotter (>60-70*F). I'd be happy to provide any additional info or pictures of the codes or the MID if this would be helpful.

Any direction or help you can provide would be most welcome!

 

p.s.
I searched the forum for similar issues before posting but didn't find anything except these two threads, which seem to be different than the issue I'm experiencing.

Strange Issues After Changing Head Gasket | PriusChat
P0345 07 Prius - Anyone dealt with this before? | PriusChat

 

Thank you for this explanation, that helps! So it sounds like there's an issue with the coil pack my mechanic installed.

 

Sorry for the confusion!
- He replaced both spark plugs and coils; based on the feedback so far, it sounds like the coils died or were faulty.

 

Thank you for this breakdown, that was very helpful! If I'm understanding correctly, Toyota designed the Prius to shut the engine off if it doesn't detect the feedback pulse from one of the coils. Based on this and another comment, it sounds like the coil pack my mechanic installed was either faulty or died and is now causing that issue. I'll check the fuses and chat with him about swapping out the coils or, at least, the #4 coil.

 

Thank you for taking the time to educate me and provide this run-down. The way you outlined it makes sense (in a frustrating, why-would-Toyota-leave-that-code-artifact-in sort of way) and I'll explore the problem as you suggest.