Reads −40°F, 999, open-circuit or a value that will not move.
1
Open or shorted RTD / thermistor
medCheckDisconnect and measure the element: a PT100 reads ~100Ω at 32°F, a PT1000 ~1000Ω. Open circuit shows infinity.
FixReplace the sensor; check the wiring for chafe and moisture in the connector.
ToolsMultimeter
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2
4–20 mA loop broken or mis-scaled
medCheckRead loop current in series: 4 mA = sensor zero, 20 mA = full scale. Under 3.6 mA is a broken loop.
FixRepair the loop or correct the scaling parameters in the controller.
ToolsMultimeter with mA range
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3
Ground loop / VSD noise on the signal cable
medCheckNote whether the reading is only unstable while the drive runs; confirm the shield is grounded at one end only.
FixRe-route signal cable away from motor leads, use shielded twisted pair, ground the shield at the panel end only.
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4
Blocked or wet pressure sense line
lowCheckCrack the sense line fitting and check for oil or water; compare against a test gauge.
FixClear and re-route the line so it drains back; add a snubber where pulsation is high.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.