PLC Repairs and Troubleshooting Tips

mitsubishi plc repairs

Is your PLC behaving erratically or suddenly just “lose” its program? Or maybe it faulted and now the red light is on and you can’t get your error codes to clear?  Does any of this mean you will be spending hard-earned money on PLC repairs?

These are common problems and concerns.  We hear these tales of woe every day from manufacturing plants and shops of all sizes.

In a perfect world you would perform monthly or bi-monthly maintenance checks on your PLCs.  But even if you manage to do this, many of the common PLC problems are not caused by lack of maintenance.

Machine age and environmental conditions are often at the heart of the problem.  Assuming you have ruled out obvious problems such as loose cable connections, here are a few other common PLC problems to look for:

  • Power fluctuations or surges
  • Ground wiring problem (look for loose or corroded connections)
  • Over heating
  • Blown power supplies
  • Blown I/O cards
  • Bad I/O channels
  • Bad cables/loose cable connection
  • Electro-magnetic interference (EMI) or Radio Frequency interference (RFI)

However, one frequent PLC problem is absolutely a maintenance issue – bad batteries.   Yes, this really happens as it did to this Mitsubishi FX-64MR-UA1/UL (see photo) which is an obsolete PLC.  Obsolete PLCs can usually be repaired, but this may add some challenges to the repair process.

If you’ve been neglecting to replace your PLCs back-up battery (a very inexpensive item!), you are setting yourself up for some real problems.  If the red fault light indicates “battery status” you are in imminent danger of losing your program.

If you do not have a current program on hand to reload after you’ve replaced the dead battery, then you are in even more trouble.  (WARNING:  If the red battery light comes on DO NOT down the power! First, get the program backed up BEFORE you attempt to change the battery(s).) Whether you are the boss and/or the machine operator, this is not a situation you want to find yourself in.  Ever.

In almost all of the above mentioned cases, you will need to remove the PLC unit and have it repaired.  If you need PLC repairs check first to see if your equipment is still under warranty.  If so, contact the OEM about repairs.

If it is out of warranty, contact an independent industrial electronic repair center to find out if they are a good fit for your company and your particular PLC and electronic repair needs. They should offer free evaluations for your damaged PLCs and other equipment, and a free quote for repairs (no bench fees!).  They should also provide repair reports if requested and a good warranty of at least one year that covers both parts and labor.

About the Author: ACS Industrial Services is an independent industrial electronic repair center providing PLC repairs for all types and manufacturers as well as for power supplies, circuit boards, drives, servo motors, CNC equipment, encoders, monitors and touchscreens, test equipment, and much more.  Rush Service is available and Two Year Warranties cover most repairs.  The customer service team is available to answer your questions and help solve your industrial electronic repair problems. You can reach them by calling 800-605-6419 or going to