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Citroen Fan Resistor Module Repair

By on January 3, 2018
transistor fet shorted










A common fault in the heater system on Citroen, Peugeot & Renault vehicles is the so called ‘smart sense’ heater fan module. It uses a computer controlled Pulse Width Modulated signal to vary the voltage applied to the fan motor.

citroen fan resistor module repair

A customer brought in one that was not working – he had replaced it with a new one, but having several Citroen vehicles, wanted to use the repaired one as a spare for the others.

A good visual inspection quickly showed the fault area.

fan resistor module repair

These are the 2 terminals that power the fan motor. As you can see from the size, they carry a lot of current. However, if you look closely, they have been overheated – even to the extent of melting the solder holding them onto the circuit board and actually pulled down from the board!

No doubt arching due to a poor connection occurred and resulted in these burnt, pitted terminals.

how to repair citroen fan module

It was decided the best course of action was to remove the connector entirely, clean up the terminals (I had no others that would replace them), resolder them and look for any other damage done to the module.

how to fix citroen fan module

fan module connector

After straightening, cleaning and repairing the terminals, they were reassembled into their connector.

The connector was then resoldered onto the circuit board.

That’s one problem solved, but was any other damage done?

Further investigation showed the power Mosfet was faulty and after removal from the board, evidence was shown on the heat sink. A burn mark indicated that the Mosfet had failed due to excessive heat.

module fan repair car

You can imagine the amount of heat that is produced by this Mosfet by looking at the size of the heat sink!

It was decided that a modification was in order. The original Mosfet had the specifications of 55V, 75A. An upgraded Mosfet of 55V, 110A was installed. This should help with higher current loads.


Removal of the thermal compound was quite a task as it had been baked on by the excessive heat.

thermal compound

The Mosfet needed to be removed from the heat sink during soldering as the heat from the iron was dissipated and not allowing the solder to melt onto the board.

Once this was done a good solder joint was completed. After the excess pin length was trimmed and new thermal compound added, the Mosfet could then be clipped back into place.

transistor shorted

But the question remained, what caused this fault in the beginning?

A cabin or pollen filter is installed in the heater/cooling system to provide clean air to the passengers inside the vehicle. It has been stated that the air inside a vehicle is actually 10 times worse that fresh air outside the vehicle. This is due to recirculated air inside the cabin that encourages airborne contaminants including fungus spores, dirt, pollen, dust (or any other nasty stuff small enough) that can cause health issues like asthma, allergies, nausea, headaches and dizziness, just to name a few. Therefore regular replacement of the cabin filter is vital for passenger health. However, more is involved – it also is vital for fan motor health! What do I mean?

As in the case with this damaged fan module, the cabin filter had not been replaced for a long time. Due to a blocked filter, the motor had to work harder to draw air into the cabin. This mechanical work is converted into higher current draw and eventually damages the Mosfet. I have even seen cases when the motor itself burns out due to the excessive current and heat being produced.

cabin filter

The customer was advised to check the female side of the connector for damage on the motor terminals as well as the need for regular replacement of the cabin filter.

If you are interested in this repair, you can see a video on my channel following the link below:


This article was prepared for you by Mark Rabone from Australia.

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Note: You can check out his previous repair article below:





  1. Albert van Bemmelen

    January 3, 2018 at 6:16 pm

    Very informative and nice detailed pictures taken from the repair Mark!
    I guess that cleaning those air filters is never done by any mechanic?

    • Mark

      January 3, 2018 at 7:11 pm

      Hey Albert,
      Many mechanics don't bother replacing these filters. I don't understand why though, because it is good business practice and also looks after the health of the vehicle and the customer. They are listed to be replaced on a regular basis in the customers service schedule.
      One of my pet hates!

  2. Chris

    January 3, 2018 at 8:45 pm

    Hi Mark!
    This is an excellent repair guide!

    I deal also with this type of modules. The most of time on BMW cars.
    I don't know why in my area the BMW's are the most impacted with such of problem, but they are.

    I would make a small suggestion to some world of yours, so we can discuss about.
    Here is the phrase you wrote "Due to a blocked filter, the motor had to work harder to draw air into the cabin."
    So, I'm not familiar with this conclusion.
    Almost all vehicle have a function to complete seal the outside air to enter into the cabin but the fan should work.
    You know what I mean?
    In that case this part would burned out in several sec or maybe a min or two.

    I like to investigate, and my primary job is IT reverse engineering and investigation. I done some investigation on this pars and how the users are use this function of the fan.

    1. These mosfets are really a low cost parts what is used in the vehicle industry but it last maybe 5-10 years.

    2. The on-state resistance of the used mosfet's are high and I assume here is the point of death.

    Almost in all cars where I repaired this unit i saw the fan switch was on stage 1 or maybe 2.
    That means, the mosfet have to dissipate lot of heat because the fan have to spin on low speed.
    The user just crank the car, the fan goes on for some time but the voltage drops for a few sec (until the engin is running) and the voltage at the fat also drops to critical level.
    In that scenario (what I also simulated on several cars) the unit goes very hot.
    I assume, using the fan system in this way is a vary bad think, and will kill the fan speed control module.

    If we put in calculation also a worse fan motor commutator + bad brushes + a voltage drop on the gate + high on-state resistance of the mosfet we have a welding machine.

    So, my opinion is to put a mosfet with a very low on-state resistance, which will result in a cooler unit even if the fan request more power in some situation.
    I tried also to use original mosfets from scrap parts, but at the end the better result was to using a mosfet with smaller on-state resistance.
    The unit with the new mosfet never returned.

    A bigger amperage didn't mean the mosfet can handle the requested current by the fan motor when the Ug is under the needed.
    That mosfet will also go hot and die by time.

    My best regards.

    • Mark

      January 4, 2018 at 5:59 am

      Hey Chris,
      I can see clearly by your articles that you know your subjects well! I may have been incorrect in suggesting that a higher current mosfet would help with the heat dissipation. I can see by what you are saying that the on-state resistance would make the needed modification. From other testing that I have done in the past, I have found that motors that have a higher mechanical resistance (or have to work harder due to dry bushes or perhaps a fan having to draw air through a blocked filter increase the current. Starter motors are a classic example. When the bushes are worn, the current increases. Although I haven't done testing on a fan motor, I assumed (you should never assume!) that the result would be similar with increased current draw.
      Thanks very much for your explanation - we hobbyists really appreciate the details you and others provide.

  3. doraiswamy SR

    January 3, 2018 at 8:48 pm

    what tht lizard doing while u are repairing!!!!..
    nice article and happy repairing

    • Mark

      January 4, 2018 at 5:49 am

      Hey doraiswamy SR,
      The gecko has been part of my repairs for many years now - just to see if people notice him. He is actually not very good at repairs and that's why he just watches.....

  4. Gary Gemmell

    January 4, 2018 at 6:10 am

    Nice work not many people would repair these things , nice to see some of us are still environmentally friendly Mark!

    • Mark

      January 4, 2018 at 10:33 am

      Thanks Garry,

      And it helps me practice my electronic skills!

  5. Ulises Aguilar Pazzani

    January 4, 2018 at 11:51 am

    happy New Year Mr Rabone , grate repair the is the way repairing like reading Your articles

    • Mark

      January 4, 2018 at 9:33 pm

      Thanks Ulises Aguilar Pazzani.

  6. George Nutzul

    January 4, 2018 at 2:36 pm

    Congratulations on a well written article. As an electronics technician with 51 years experience (the last 21 years in the fan/blower development division of a military/aerospace company) it has been my experience that blower motor current DECREASES as the filter becomes more plugged with dirt. Take a vacuum cleaner, hold your hand over the hose inlet, and the motor speeds up. You may think that since the motor speeds up, it is working harder and drawing more current. Actually the OPPOSITE is true... if you measure the input current of an impeded blower motor the current DECREASES, since the impeller is doing no actual work, but just spinning in a laminar airflow. Long story short, I believe your problem is not caused by a plugged filter, but rather by cheap mosfets. If an auto company can shave $1.00 off each car by using underrated parts, and builds 100,000 cars, that's a $100,000.00 savings for the stockholders. My apologies for a wordy reply... you did a great job fixing the PWM module!

    • Mark

      January 4, 2018 at 9:42 pm

      Hey George,
      Thanks for your reply. You are reinforcing what Chris said in his comment above. I obviously assumed incorrectly that the current would increase with the work load increasing, similar with a starter motor on an engine.
      Yes you are right, the automotive industry is happy to save a cent if it will make their investors happy! We are now finding a lot of aluminium wiring in vehicles, replacing the copper. They say that it is to improve weight and therefore emissions, but we all know that it is about the money. It is said that they can save up to $20/car - that is a huge saving. The problem is that it is less conductive, needs to be thicker, has less tensile strength, expands more, generally needs to be ultrasonically welded to join and the list goes on.
      So, fitting cheaper mosfet that will fail makes sense.
      Thanks again for your comments.

  7. Yogesh Panchal

    January 4, 2018 at 6:07 pm


    Big CAR companies are sometime not up to the mark in some R&d AREA.
    As you stated about the cause of "Due to a blocked filter, the motor had to work harder to draw air into the cabin"

    This type of design not only accumulate dust and blocking air to get suck to another side and this process is generating heat due to load on motor. same principle is applied for house hold Vacuum cleaner i am having one of good brand with the same problem it gets very hot if you keep it on more then 3 minutes.

    • Mark

      January 4, 2018 at 9:44 pm

      Hey Yogesh,
      Thanks for your comments. Yes, these motors will get a lot of dust in them if the filter is not replaced on a regular basis.
      That's why maintenance is so important!

  8. Robert Calk

    January 4, 2018 at 6:34 pm

    Good job, Mark. I have a Weller D550 240/325W soldering gun for those heavy duty soldering tasks. I got a good used one on eBay. It works great. If you get one, get an older one that has screw nuts that lock down the ends of the tips. The new kind are said to be cheap and flimsy, but I haven't tried one.

    • Mark

      January 4, 2018 at 9:47 pm

      Hey Robert,
      My soldering iron was put through it's paces, mainly because I forgot about the heat sink!
      I guess that's what the sink is for - to dissipate heat - which doesn't help when you are soldering!

      • Robert Calk

        January 5, 2018 at 7:33 am

        Piece of cake for my Weller!

        • Mickeysat17

          February 1, 2018 at 11:58 am

          I’ve got that same 325 watt weller. A real work horse use it even on hvac pipes! Works like a hose!

  9. Chris

    January 5, 2018 at 5:16 am

    This article is a great writing, lot of nice photos with clear moderated focus on what is mentioned.
    The video is also a great part of this doc.
    I like the concept how this is done and the discussion too.
    This type of problem and repairing can be viewed from many angle of viewpoint
    and all can be true.

    Well done and keep doing so.
    I like your writings.
    My best regards.

    • Mark

      January 6, 2018 at 11:11 am

      Hey Chris,
      Thanks again for your feedback. I really appreciate this forum where we can share, learn and encourage. We all have our strengths and weaknesses & it is great to be able to be among so many that are interested in helping one another to become better at electronics.
      I enjoy including a video as well, as that is how I learn the best, by reading and then seeing the process being done.

      • Chris

        January 7, 2018 at 10:06 pm

        What software you use for editing your video?

        My best regards.

  10. Parasuraman S

    January 5, 2018 at 9:30 pm

    Very good and informative article, though not of my line or work! Good job!

    • Mark

      January 6, 2018 at 11:11 am

      Thanks Parasuraman!

  11. Albert Hoekman, Holland

    January 21, 2018 at 12:12 am

    Hi Mark, thanks for the nice article and the nice pictures.

    I drive a Renault Scenic wich has the same resistor module. I replace every year the pollen filter but last year the module still become faulty.
    What I did was replace the pins from an old rear light module that has te same pins. After half an hour soldering the problem was over.
    The only big problem was to find and dismount the module... I had to take a great part of the car's dashboard to dismount the module. All the work took about half a day! I'm glad I am retired (grin).


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