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Coolant Temperature Override (CTO) Switch

NOTE: Engine coolant temperature must be below 100°F (38°C) to perform this test.
  1. Check vacuum lines for leaks and correct routing. Disconnect vacuum line at backpressure sensor (if equipped) or at EGR valve, and attach this line to vacuum gauge.
  2. Operate engine at 1500 RPM. No vacuum should be indicated on gauge. If vacuum is shown, replace CTO switch.
  3. Idle engine until coolant temperature exceeds 100°F (38°C) on 4-cylinder engines, or 115°F (46°C) on 6-cylinder and V8 engines.
  4. Raise engine speed to 1500 RPM. Ported vacuum should be shown on gauge. If not, replace CTO switch.
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