Cummins/Onan — Thermostat test
Testing & diagnosis
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Procedure
- Remove thermostat from cylinder head.
- Heat a pan of water to approximately 150 F and check with thermometer.
- Suspend thermostat in water; it should start to open at 150 F.
- Remove after fully open and allow to cool in air; should close shortly.
- If it sticks or does not operate, replace it with a new one.
- Always install a new gasket when replacing thermostat.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Thermostat should start to open at 150 F
Source: Cummins/Onan service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.