U3574Coolant Mixing Valve
- Generic OBD-II code · Network
- Turns on the check-engine light
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- Fuel cell system warning, MIL on
- Stack temperature swings or stays too high under load
- Reduced power, slow warm-up in cold weather
- Likely causes
- PossibleValve element sticking from deposits, worn seal or unsuitable coolant
- PossibleActuator gear or motor worn, valve moves sluggishly
- RareInternal leak, coolant passes the closed port
- RareAir in the stack coolant loop after service distorting the temperature response
- Repair
- Hard · Usually a job for a workshop.Typical cost: €250 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.