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U3574 Coolant Mixing Valve

Check engine lightOn
EmissionsNot relevant
Drive cycleRequired
Limp modePossible

U3574 serves as the catch-all for the coolant mixing valve when the trouble lies in its behaviour rather than its wiring: it sticks, moves too slowly or misses the commanded split between radiator and bypass. Temperatures reveal it, because GM sets stack outlet temperature with this valve and the inlet-to-outlet difference with pump speed, so a jammed valve upsets both. Toyota plans for the case: before each start the valve runs to a fully open end position to reset its zero point, and in one variant it is left open toward the radiator at shutdown, so a valve that seizes later still cools the stack. Hyundai patented a lower-friction plunger seal for its stack coolant valve against leaks and wear. No maker documents U3574 publicly. Expect a warning, swinging temperatures and less power.

Likely causes

Valve element sticking from deposits, worn seal or unsuitable coolant
Actuator gear or motor worn, valve moves sluggishly
Internal leak, coolant passes the closed port
Air in the stack coolant loop after service distorting the temperature response

Symptoms

  • Fuel cell system warning, MIL on
  • Stack temperature swings or stays too high under load
  • Reduced power, slow warm-up in cold weather

Repair estimate

DifficultyHardWorkshop job
Parts and labour€250–1,500
Time1–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

coolant mixing valve

Related codes

U3573U3575U3570U3571

References: patents.google.com, patents.google.com, patents.google.com.