Fixing P0071 Fault

[Father Ted]

So. I had my first error code come up last week.
Being more used to old fashioned spannering, rather than ECU and code readers etc, I had no way to identify it. All I had was the engine management light on.

I bought a code reader and it was a P0071 fault.
This is an Ambeint Temerature Sensor fault.
The sensor is in the passenger (UK Spec) mirror.
There is another sensor in the driver morror, but that is for the dash display.

I checked for 5v at the sensor and that was there, so I had an idea that the door module was ok.
I did some research on line and managed to find a sample of resistances for this sensor.
They are as follows, along with the temperature for each reading.
68 Deg. F - 2424 ohms
77 Deg. F - 1941
86 Deg. F - 1513
104 Deg. F - 1034

Mine was nowhere near that. At 6d C mine read about 180 ohms.

I thought of going round the local breakers looking for one, but decided to go to Maplin and buy a new Thermistor. I had to take a rough guess at the resistance as they didn't have resistance graphs across a temperature range ( basic stuff for this type of item!).
I bought 2 x 4k7 Thermistors
http://www.maplin.co.uk/p/47k-ohm-bead-thermistor-fx21x

As it turned out, one was too high resistance, but when I put them in parrallel I got pretty much the resistances shown above.

I fitted them to the car, reset the error and (so far) it is working fine.

Here's how I fitted them: Just in case anyone else wants/needs to do this.
Buyer beware, though. These are complicated cars and I accept no responsibility if you try this and it all goes wrong.

[Father Ted]

Firstly, remove the door card.
The first step is to take the cover off the handle. Gently lever this off with a screwdriver, or, preferably, a strong plastic lever.




Once you've done this, undo the two screws with a Torx key. Use a bit of Bluetak on the end to hold the screw when doing the top one.


Locate the door rivets


and gently push the centre in.


Pop off the black plastic triangle covering the mirror reaining bolt.

Gently prise the door card off and place the rivets carefully. Seperate the iner from the outer of the rivets.
Don't try to fully remove the door card. The door module is attached to the car and (I believe) if you unplug this with the battery connected, you get an airbag error.

You will see this green plug near to the top of the door. Un plug it and hold the free end up, allowing you to temporarily refit door card.


Remove the retaining nut holding the mirror on.


Then take the mirror to somewhere warm and comfortable!
On the bottom of the mirror you will see the pop in panel with a nipple on it. This is the sensor plate. I challenge anyone to remove it with leaving tool marks!
Once released, carefully prise out the original sensor. THIS WILL BREAK IT!
I tried to get it out carefully, but the resin used to hold it in place is strong.


I then cut through the resin to expose the wires and soldered by thermistors in parrallel onto the ends


I placed them into the plate and filled it with hot melt glue.



Refitting, as they say, is the reverse!

[Polo on]

Great work, and detailed pictures with explanations Father Ted, however, has anyone considered replacing these sensors with 2 in better locations? Namely not passing through the 2 parts of the car that move the most, and get slammed the most.
I have a problem with the dashboard temperature display and would like to find the wires inside the car so I can cut them off and mount a replacement sensor (maybe made like yours) but in another location e.g behind the bumper where most cars have theirs.

Anyone know what colour the wires are are where they can be intercepted?

[Father Ted]

Unfortunately I can't remember the colours.
Good idea about mounting it elsewhere, although, I'm not sure it was a mechanical issue, it was just electrical failure of the thermistor.