Oso water heater shows E13 and may shut down; often associated with high tank pressure or discharge from safety valve.

Safety first: Hot pressurized water is dangerous. Isolate power and allow the system to cool before working on pressure components.

• Check actual pressure: If a gauge is fitted on the Oso cylinder or nearby, compare its reading to the control display. If the display shows high pressure but the gauge is normal, suspect the sensor.
• Inspect expansion vessel: On unvented Oso systems, check the expansion vessel pre‑charge with a suitable gauge (with system depressurized). Recharge or replace if the diaphragm has failed.
• Verify discharge pipework: Ensure the pressure relief valve discharge is not blocked or frozen, which can cause dangerous pressure buildup.
• Replace pressure sensor: If fitted and faulty, isolate and drain to relieve pressure, then remove and replace the Oso pressure sensor with the correct part, using appropriate sealing.

Any persistent over‑pressure condition should be investigated by a qualified unvented cylinder engineer.