Question – How do I troubleshoot a code “88” fault code?

Answer – There are several steps to troubleshoot the code 88 and the 72 alarm. Use the steps below to resolve this issue:

  • Confirm that the sensor is plugged into the control box.  You will need to disconnect main power and open the front cover of the pump to do this.  If it was not plugged in, plug it in and run the pump and see if this took care of fault 88.
  • If fault 88 still exists, make sure the sensor cable plug connections are secure and clean, and the sensor cable is installed correctly. Pay attention to the small pins in the sensor connector:
  • If there is still an issue, check the voltage across these terminals of the sensor cable (see below). If the reading is 0V, the cable is either not connected or damaged.  Re-connect or replace the sensor cable (P/N 98700244)   If the reading is between 4.65V and 4.88V, then it’s OK. If it is outside this range, then the entire control box needs to be replaced.

Below are the part numbers for replacing the control box. Please note that these will be a special order from your wholesaler.

  • If you’re still having an issue the, the sensor is likely plugged. Turn off power to the pump and drain it. Remove the sensor bracket and pull the sensor out of the pump housing. The hole, and the one on the opposite side (see pic below) should be clear, and slightly reflective. If the sensor holes are plugged with debris, or you can see through to the other side, the sensor needs to be replaced with part number 98471249 or pull out the rubber cap and carefully clean. Run the pump and see if this took care of fault 88.
  • If the fault still exists, confirm correct cap is being used – see below. New cap is 7/8″ long, old cap is 5/8″ long. The new cap is sold as a service kit (P/N 98708139, qty of 10). Sensor cap should be re-inserted “nose down” see (see pic below). Run the pump and see if this took care of fault 88.
  •  If the fault remains, remove pump head to inspect the ports in the pump housing. Make sure they are clear of any debris. Make sure to remove the sensor first, then run a wire from the inside hole of the volute to remove ant debris. To remedy any future debris issues, install a Spirotherm VDT series combination air and dirt eliminator.  
  • If fault 88 still exists, call your local Grundfos rep agency.