If element resistance tests good but the water heater isn\'t producing hot water, the thermostat is the next suspect. Thermostats control when each element receives power. A stuck-open thermostat means the element never energizes.
How Marathon thermostats work
Marathon uses two thermostats (same architecture as standard electric tanks):
- Upper thermostat (with ECO): controls the upper element AND switches power to the lower thermostat. Includes Emergency Cutoff (ECO) red reset button
- Lower thermostat: controls the lower element only. No ECO function
Operation: upper element heats first (priority). When upper section reaches setpoint, upper thermostat opens, transferring power to the lower thermostat. Lower element heats. Only one element fires at a time (non-simultaneous).
Diagnostic procedure
1. Verify ECO not tripped
- Open upper access cover
- Look at upper thermostat — find the red ECO reset button
- If button is popped (visibly extended), ECO has tripped. Diagnose root cause (overheating, stuck thermostat) before resetting
- If button is flush, ECO is not tripped. Continue diagnosis
2. Continuity test — upper thermostat
- Power OFF at breaker; verify with voltage tester
- Disconnect leads at upper thermostat
- Multimeter on Ω or continuity (beep mode)
- If tank water is below setpoint (cool): upper thermostat should close. Probe across L1-T1 = continuity. Across L2-T2 = continuity (transfers to lower)
- If tank water is at setpoint (hot): upper opens, lower path closes
- Set thermostat dial fully clockwise (max) to force closure for test
3. Continuity test — lower thermostat
- Same procedure
- Lower stays closed when section temperature is below setpoint
- Should show continuity across terminals when cool
4. Interpret results
- Continuity present when expected: thermostat is good. Look elsewhere
- Open across terminals when should be closed: thermostat stuck open. Replace
- Continuity always present: thermostat stuck closed. Will overfire and trip ECO. Replace
Replacement procedure
- Power OFF at breaker; verify with voltage tester
- Remove access cover, fold back insulation
- Photograph existing wiring
- Release spring clip and lift old thermostat off the tank shell (no draining required)
- Clean shell contact surface — any residual insulation or paint affects readings
- Press new thermostat firmly against shell — clean contact is critical
- Reconnect leads per photo
- Verify dial position (factory 120°F is typical)
- Replace insulation and cover
- Restore power
Critical: shell contact
The thermostat reads tank temperature by direct contact with the steel shell. (Wait — Marathon has no steel shell? Correct — Marathon\'s composite tank has a metallic temperature contact plate where the thermostat clip seats, specifically for this purpose.) Air gap between thermostat and contact plate = wrong readings = over-fire or under-fire.
Cost
- Upper thermostat (with ECO): $15-30
- Lower thermostat: $10-20
- Within 6-year parts warranty: Marathon covers part
- Plumber labor if hired: $80-150
Marathon thermostat compatibility
Marathon uses standard industry surface-contact thermostats. Universal upper thermostats (with ECO) and lower thermostats from Honeywell, Therm-O-Disc, or Camco fit Marathon. OEM is preferred for warranty compliance, but universal equivalents work identically.
Bottom line
Thermostat test follows element test in diagnostic priority. Stuck-open thermostats produce "lukewarm or no hot water" symptoms even with healthy elements. Quick continuity test confirms before ordering parts. Universal thermostats interchange with Marathon at correct ratings.