Block 02 (outdoor PCB). TE (outdoor heat-exchanger) or TS (suction pipe) sensor is out of place, disconnected, shorted — or the two are misconnected into each other's connector. The manual warns 18 can also appear when the 4-way valve is mis-operating, because TE/TS then read an abnormal relationship.
Safety: Safe isolation before disconnecting sensors or working on the PCB.
UK models this code applies to
From the Toshiba service literature we hold (Toshiba service manual SVM-19001-3 (Seiya) / SVM-20015-4 (Shorai Edge) §11-4, Table 11-4-1) — not a scraped list.
18 names a symptom, not a root cause. In the app, the same code opens a guided investigation: what to measure first, the expected values for your exact model, and an honest verdict — including “nothing in this machine is broken” when that's what the readings say.