Firstly, thanks for having us here (My son and I).

Just on the weekend bought my sons first car, a '99 Forester GT with 297ks on the clock, it has a few mods and an issue or two already.
1. Car no longer starts, turns over/cranks, but will not fire.
We have one key and when its inserted, Immobilizer light is on (not flashing). Check Engine light is flashing.
Car did run till we got it home..

We did fill it with 91RON ( we didnt know better) but have been assured this should not stop car from starting.

2. It has fitted (undisclosed by the former owner) an HDI EBC-R set at around 45% on the dial? Any idea how much boost this will be passing, and what impact if any, short and long term, this may have on the engine.
Boost gauge saw 20psi boost when we were on the way home.

My son will be learning to drive in this car within the next 5-7 weeks.
Thanks in advance for any assistance you can offer.

well , that response was underwhelming.
was hoping that someone here might know something about either of the issues, but I guess its a closed shop...

fuel and spark make sure its got both (check for dead fuel pump) yep 91 won't stop it firing. 20 psi with 91 ron not a good combo i.e. detonation pinging etc. Not cranking could be from dodgy immobillzer or ignition there is a fair few. I always start with the basics make sure its got fuel pressure and spark.

Oh noticed you live south of the river if you get really stuck try Stechnic in cannington subaru's are there thing.....

Agree...20 PSI is too much. The car seems to have gone into a self-protect mode. Best you give Doug or Damien at S-Technic a call...they are not too far from you in Cannington.

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No Subarus currently owned

Oops, need to clarify.
the car DOES crank/turn-over, but will not fire.

Dead immobilizer antenna has been diagnosed, bit in checking with multimeter last night, its still intact.

at $340 and 3-4wks ex Japan for a new copper spool inside a plastic housing, I am trying to eliminate other issues before I drop the coin on a new unit.

re self protect mode - car drove home (when we saw the indicated boost) and was then cycled three or more times locally over then next hour during my sons first lesson (as well as showing the in-laws) so wasn't an immediate cease of action.

Did you sort this out