The Telegan Sprint V3 can sometimes display differential temperatures incorrectly.
This is caused by one (or both) of the measurement points not being earthed well which causes the unit to not calculate the differential correctly. This was not an issue with previous versions of this device.

Here the differential temperature is being measured on an Ideal Logic at room temperature and the appliance has not been running, the differential between flow and return is being displayed as 34°C. This is incorrect and could potentially lead to a misdiagnosis.

If when trying to carry out a temperature differential the readings appear inaccurate then further investigation will be required. It will be necessary to check the earth continuity between the measurement points and earth (as with preliminary electrical checks/earth continuity). A resistance reading of <1 Ohm should be seen between the points you intend to attach the temperature probes and the main earth. If the resistance is greater than this the temperature readings you receive will be inaccurate (a larger resistance causes greater inaccuracy), in extreme cases differentials of -250°C have been reported.

To overcome this issue either use a different device to measure the temperatures or alternatively (where possible/safe) use a temporary continuity bond or similar to connect the measurement point(s) to earth as shown below. This will then correct the readings displayed and with the test points correctly earthed the differential now shows as 2°C at room temperature.