How to Replace Temperature Sensors on Bosch Tankless - Instructions

How to replace temperature sensors on Bosch tankless water heaters? When is the right time for the replacement?

Temperature sensors on Bosch tankless water heaters are factory installed to measure the incoming cold and outgoing hot water temperature, compare and ensure that the correct set temperature of hot water is met.

If everything is ok with the sensors, the unit will accurately measure water temperatures and provide the desired temperature of hot water. If, for example, temperature sensors have loose connections, are covered with the deposits and are corroded, the measurement will be incorrect, resulting in temperature fluctuation, uncomfortable showers, cold water instead of hot or warm, followed by the error code display, sometimes.

Bosch tankless water heaters have two temperature sensors, the inlet sensor located on top of the water valve and the outlet temperature sensor, found on the outlet pipe. The inlet sensor measures the cold water, while the outlet sensor - hot water, both working in correlation. If one of the sensors doesn't operate properly, the following steps have to be taken by using the proper tools:

Required tools

  • Phillips-type screwdriver
  • Multi-meter
  • Thermometer
  • Cloth


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  • Press the reset button to clear the error code, if present.
  • Turn the water heater OFF.
  • Remove the front cover of the heater by taking out two screws.
  • Loosen the three screws of the control unit that holds the control panel in place.
  • Locate the water valve and the inlet temperature sensor; these are at your right and right above the control unit when looking at the front of the heater.
  • Locate the hot water outlet pipe and its sensor; these are on your left.
  • Open the hot water tap while the water heater is turned off, let it run for several minutes, and measure cold water temperature.
  • Upon closing the hot water tap, press and hold the program button (P) on the water heater and put the unit in the ON position.
  • The "188" code appears on display. Release the P button. Now the screen should read P2.
  • Press the "+" sign on the control unit to get into the P4 diagnostic mode.
  • Press the P button again, and the E sign should appear.
  • Scroll through the diagnostic modes by pressing "+" or "–."
  • Access the inlet water temperature reading by scrolling to mode 1d.
  • Scroll to mode 2d to access the outlet water temperature reading.
  • Press P to see diagnostic information and check the readings.
  • The diagnostic temperature reading should be within +/-5 F of the temperature reading at the hot water tap.

If the reading is not within +/-5 F do the following:

  • Put the water heater in the OFF position and open the hot water tap for several minutes. Unclip the wire connection from the sensors attached to the water valve and hot water outlet pipe and use the multi-meter to measure the resistance (in ohms) inside the wire connectors.
  • The resistance readings could be found here and match up with the reading at the hot water tap.
  • If readings are different, the sensor (s) may need to be cleaned and, if defective, replaced.
  • First, try to clean one or both sensors. Unclip it from the pipe or water valve and use the cloth to wipe clean both sides (and polish) where it makes contact. Either you are putting the old or new clip back; make sure the sensors are correctly installed and make good contact.
  • Resume the power and test the water heater.

Check out a detailed guide here.

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