Photo: OWS Photography, CC BY 4.0, via Wikimedia Commons
2 faults documented by at least two sources, listed by version then by mileage at onset when known.
2 known faults
Several IS 300h owners report 12-volt batteries replaced repeatedly, or a hybrid system that fails to start because of low voltage; one case involves a parasitic current measured by the dealership.
IS 300h owners report a faulty water pump (bearing leak on a 2014 car; failure at 148,000 km and 10 years on a 2013 car, not covered by Lexus). These are isolated reports rather than a widespread defect.
No recurring fault documented for: IS 200t.
An OBD scanner plugged in under the steering wheel shows stored fault codes, even with warning lights off: useful during the test drive.
Compare prices of parts for the Lexus IS III (XE30) (timing, brakes, filters) to estimate the repairs to plan for.