This is a shocking answer but one of the heaters may have a break in it's heating element liner and be the culprit. You have to cut the AC power to the heater and then remove both sides of the electrical connections on the heater elements. ( Most times there are two) with a ohmmeter there should be no connection or leakage from the heater element electrical terminials to the tank metal
When a 250V, 2500W water heater is connected to a mains supply with a 5A fuse, the fuse will likely blow. This is because the heater draws approximately 10A (calculated by dividing the power, 2500W, by the voltage, 250V), which exceeds the 5A rating of the fuse. As a result, the fuse will interrupt the circuit to prevent overheating or damage, preventing the water heater from operating.
Replace fuse #36 in the inside fusebox.See "Related Questions" below for more
Ck the heater fan motor resistor .
in the dash but check the resistor 1st the blow e.z. it is in engin comp. passinger side on heater core lines it looks just like a water heater fuse
The fuse for the blow heater (or blower motor) on a Peugeot 406 is typically located in the fuse box, which is usually found under the dashboard on the driver's side or in the engine compartment. You can identify the specific fuse by checking the fuse diagram, which is often printed on the cover of the fuse box or found in the owner's manual. Make sure to replace it with a fuse of the same amperage to avoid electrical issues.
short circuit in the circuit
it can be caused by a bad alternator
One of your wires that have to do with the Reverse light are touching a another wire that they shouldent be touching ( + is touching a - ) it makes the fuse blow...if any changes made recently with any wires rewire or check if correctly wired
If the heater fails to "blow," then the fan motor (situated on the fire wall inside the engine compartment) is disconnected or has failed and should be reconnected or replaced. But, before either of those, check the heater fan fuse, replace if blown, and re-try the heater.
If the fan is not blowing, a fuse may be the cause. If there is just not warm air being blown by the fan, check your coolant level in your radiator. Low level of water and coolant will cause the heater to blow cooler air.
in the water heater
Defective blower motor, blown fuse, or a defective Blower Motor Resistor Pack.