是否为使用不当引起的故障，如属于这类故障，则根据使用情况可初步判断出故障类型、发生部位。常见的使用不当包括供电电源故障、端子接线故障、模板安装故障、现场操作故障等。
如果不是使用故障，则可能是偶然性故障或系统运行时间较长所引发的故障。对于这类故障可按PLC的故障分布，依次检查、判断故障。首先检查与实际过程相连的传感器、检测开关、执行机构和负载是否有故障：然后检查PLC的I/O模板是否有故障：最后检查PLC的CPU是否有故障。
在检查PLC本身故障时，可参考PLC的CPU模板和电源模板上的指示灯。
采取上述步骤还检查不出故障部位和原因，则可能是系统设计错误，此时要重新检查系统设计，包括硬件设计和软件设计。
Narda DC to 4.0 Ghz, 10 dB, 1 W, 50 ohm, SMA Fixed Coaxial Attenuator
Microlab / FXR W501C (WR-112) 7.05 to 10 GHz, Waveguide Load / Termination
Nikon CFWN15X/14 15x Magnification, 14 Focal, 16.7mm, Eyepieces, Set of 2 pcs
Vickers Pilot Valve DG4S4W018C50 #30387
Waveline 732-1 (WR-62) 12.4 to 18 GHz, Choke Flanges, 90° Waveguide E-Plane Bend
Allen Bradley 500AOB930 NSFP **GENUINE** AB
Shinwa Control Panel S/W Operator Scratch & Dent #70093
HP 50 GHz UTG Mixer Assy. For 85110A, 8760C, E7340A, E7350A
GE CR115E4441D2 ROTARY LIMIT SWITCH
Vickers Directional Control Valve F3DG5S82AL2EMWB20 #37909
Plastic Process Equipment Visionflow Meter / Manifold Flowmeter941 #78946
Exfo FOT-20A (FOT-22AX), 780, 850, 1300, 1310, 1550 nm, Optical Power Meter
ASCO 8344C25 VALVE 120/60 *USED*
Siemens Temperature Control Board 6ES5244-3AA22 #78408
Bosch Valve 0 811 402 003 #80603
(V55-1) 1 RELIANCE ELECTRIC AW THYRISTOR
AMETEK BBVRR1RHYDLXBX90651 BARGRAPH
HP 16099A Test Fixture Adapter, for use with HP 1692A - 1693A
Leica StereoZoom 4 (SZ4) 0.7 to 3X, Microscope + (2) B+L 10X W.F Eyepieces
Bussman BDH100 Pistol Hold, I/O, Black / Red, Handle Assy., in Box
ASCO 8210G026 VALVE-Asco 8210G095VM Solenoid Valve, 3/4" NPT, 10.1W
SIEMENS 3VE4200-0CS00 CIRCUIT BREAKER-SIEMENS 40DP32AA CONTACTOR
ABB PIEDMONT BUSHINGS & INSULATORS K1 CONTROL POWER TRANSFORMER
Cincinnati Milacron Communication Module A #62552
T7006A Duct Temperature Sensor
(V10) 1 GE FANUC IC693CMM302L ENHANCED COMMUNICATION MODULE
Hobart Circuit Board 408101 #79008
Frequency West / Loral MO-111XC-48, 1500 to 1720 MHz, Microwave Oscillator
Nordson Temperature Control Board 104075E Appears #29131
HP / Agilent Board Assembly, M / Video Graphics Assembly
YOKOGAWA DXA3/F1/S13/NS-S CHART RECORDER
Broadwave Technology DC to 3.0 GHz, 100 Watts, SMA, Coax Termination
Barber Colman Control Card Rack 40BA-411BC-COO-A-00 #23896
JOHNSON CONTROLS T TEMPERATURE GAUGE
Tektronix , DC to 18 GHz, 50 Ohm, 14dB, SMA (M-F), Coaxial Attenuator
Barber Colman Communication PC Board A #23698
Solid Controls Process Control Board 24963 #5997
Cosel P600E-15, 85 to 264 VAC Input, 15 VDC/43 Amp & 645 W Output, Power Supply
Red Lion Controls PCU01000 VAC, 50 / 60 Hz, 10 VA, Process Controller
Rice Lake Weighing Systems, 1 to 50 Gram, Class 6, 9 pc Weight Set with Case
GOULD MODICON AS-P MEMORY MODULE
(8319) Schroeder Differential Pressure Switch MS11DC