Parts for your 2017 Subaru Legacy-Tx valve

2017 Subaru Legacy TX valve — what it does and when to replace it

Yes, the 2017 Subaru Legacy uses a thermostatic expansion valve (TX valve). Technical references that confirm this include the Subaru Factory Service Manual for the BN/BS Legacy/Outback platform (2015–2019) HVAC section, which specifies a “thermo expansion valve” at the evaporator inlet, and Subaru’s parts catalogues listing an “Expansion Valve Assy” for these models (commonly referenced as p/n 73520AL00A, with market supersessions possible). DENSO’s air-conditioning service literature for Subaru applications also documents a TX valve regulating refrigerant flow into the evaporator. So the TX valve is absolutely relevant on a 2017 Legacy.

The TX valve’s job is to meter refrigerant into the evaporator based on temperature and pressure, keeping the evaporator just cool enough without freezing. It maintains a small “superheat” at the evaporator outlet, improving cabin comfort, protecting the compressor, and helping the system cope with Aussie and Kiwi temperature swings. Depending on market, the car runs R‑134a or R‑1234yf, and the TX valve manages flow for either refrigerant to keep pressures in check.

It’s not a routine service item, but it does come into play when faults show up or the system is opened for major work. On the 2017 Legacy the TX valve is mounted at the evaporator in the HVAC case on the cabin side. Access typically involves glovebox/trim removal and HVAC case access, many workshops evacuate the system, disconnect the lines at the firewall, and replace the valve with new O‑rings before vacuum and recharge to the exact specification.

• Common symptoms of a crook TX valve:
  • Poor cooling at idle, then too cold at speed, or vice versa
  • Frosting on the evaporator or lines, intermittent chilly blasts
  • Abnormal gauge readings (very high or very low low‑side pressure)
  • Compressor short‑cycling or hunting
• Good practice when replacing:
  • Recover refrigerant properly, never vent to atmosphere
  • Fit new O‑rings lubricated with the correct oil for the refrigerant
  • Replace the receiver/drier (or desiccant) if the system’s been open or after compressor failure
  • Pull a deep vacuum and recharge by weight to spec, check for leaks and verify outlet temps
  • Torque fittings to the Subaru service manual spec

If the Legacy’s air‑con isn’t behaving, a TX valve stuck open or closed is a usual suspect, especially after a compressor or moisture contamination event. A tidy fix with proper evacuation, fresh seals, and a new drier will keep the A/C sweet for many kilometres.

FAQs

Does the 2017 Subaru Legacy use a TX valve or an orifice tube?
It uses a thermostatic expansion valve at the evaporator inlet. Subaru’s BN/BS platform HVAC design specifies a TX valve rather than a fixed orifice, giving better control of evaporator superheat and cabin comfort across changing conditions.

When should the TX valve be replaced on a 2017 Legacy?
It’s replaced when diagnosed faulty (stuck, restricted, or hunting), after compressor failure with debris, or any time the system has been open to atmosphere and shows metering or moisture issues. It’s not replaced on a time or distance schedule.

Do I need to replace the receiver/drier when changing the TX valve?
Best practice is yes. Any time the circuit is opened, moisture control is critical. Replacing the receiver/drier or desiccant bag helps protect the new or existing compressor and ensures stable pressures and cooling performance.