FieldBench
Bosch codes

Bosch

E4

Lockout

Temperature sensor fault

T3, T3L, T4, T5, or TF out of range.

Walk E4 in Diagnose

Sign in for the next measurement

First check

1 Which sensor is this code?

On Bosch BOVA 2.0 E4 is a specific thermistor — indoor return/pipe, outdoor coil, discharge, or ambient. Unplug that sensor and ohm it vs the OEM temperature/resistance chart. Identify which sensor the display names.

Expect: Ohms move with temperature. Not open, not shorted.

The rest of the walkthrough is Diagnose.

On the plates

  • Identify which sensor the display names.
  • T3/T4/T3L: 10K. T5/TF: 5K class — use the IDS ohm chart. Open/shorted harness vs failed bulb.

Plates that use E4

Same number, other makers

E4 on Bosch is not the same code on these plates.