A symptom is
a starting point.
A meter-first map for faults that look like bad sensors, bad PLCs, or bad luck.
The loop reads 0 mA
Power loss, reverse polarity, blown barrier fuse, or open field wiring.
Trace the fault → 02The PLC shows the wrong value
Scaling math error, input mode mismatch, or unconfigured live zero offset.
Trace the fault → 03A PNP sensor stays ON
Shorted transistor output, floating 0V common, target hysteresis, or metal chips.
Trace the fault → 04An M12 sensor drops out
Connector oxidation, loose knurled nut, water ingress, or drag-chain cable fatigue.
Trace the fault → 05The analog value wanders
Ground loop circulating current, VFD carrier noise, or ungrounded shield drain.
Trace the fault → 06Ground loop voltage offset
Potential differences between field tank earth and substation instrument common.
Trace the fault → 07HART communicator timeout
Missing 250 Ω series resistor or power supply capacitor shorting FSK audio tones.
Trace the fault → 08Modbus RTU frame CRC errors
Missing end-of-line termination, floating differential bus, or star-topology stubs.
Trace the fault → 09PLC output card transistor burnouts
Unsuppressed −400V inductive kickback spikes upon de-energizing 24V solenoid coils.
Trace the fault → 10Temperature reading decreases as heat rises
Reversed extension wire polarity creating parasitic cold-junction Seebeck errors.
Trace the fault → 11RTD temperature consistently high
2-wire lead resistance adding artificial ohm offset; uncompensated cable run.
Trace the fault →