HOW-TO: replace your MK3 heater core PDF Print E-mail
Written by Vince Waldon   
Monday, 01 September 2008

This HOW-TO outlines my notes after spending a wonderful weekend replacing the heater core in my 1994 Jetta.  Not the worst proceedure I've ever had to do, but it does take time and needs a bit of thought.
















 Tools needed:


  1. 8mm socket.. ¼” screwdriver-type handle is useful on this
  2. 10mm shallow and deep sockets
  3. 13mm socket
  4. XXX torx driver
  5. Phillips screwdriver
  6. wire cutters
  7. ¼” drill bit and drill


Parts needed:


  1. new heater core
  2. a bit of coolant
  3. 2  new steering wheel bracket shear bolts (N 905 422 01)  
  4. zip ties
  5. ¾ ” plastic plumbing elbow as a temporary heater core bypass
  6. a couple of screw-type clamps... the factoy spring clamps won't quite be tight enough













Some shortcuts that worked for me:


  • NO NEED to drain the cooling system… just work quick to plug the heater hoses with a ¾ ” plumbing elbow
  • NO NEED to vent/disconnect the air conditioning… with everything loose you can wiggle the heater core end of the heater box just enough to get the heater core out with the rubber part of the A/C hoses taking up the flex.
  • NO NEED to remove the rebar.  Loosening it and dropping the passenger side down is enough and saves some wiring disconnects on the driver’s side
  • NO NEED to disassemble the heater box…as above, the secret here is to loosen everything and then pry carefully on the heater core side.  Leave the heater box clips alone !!


Under the hood:


  1. disconnect the negative terminal of the battery
  2. remove the two piece plastic gutter guard
  3. remove the windshield wiper arms and motor assembly
  4. remove the two heater hoses… working quickly, plug ‘em with a ½ inch plastic plumbing elbow as a temporary bypass
  5. (air conditioning) remove the vacuum hose in the middle of the two heater hoses
  6. (air conditioning) remove the two 10mm nuts securing the air condenser side of the heater box to the firewall.  These might be there for non-airconditioning boxes as well… dunno ‘cause I have air.
  7. remove the two 10mm nuts securing the heatercore side of the heater box.  They are behind the metal heatshield glued to the firewall.  One is accessible thru a tab in the heatshield down by the tin heatshield for the brake line, the other you will find by prying the metal heat shield away from the firewall by where the heater hoses come out.
  8. remove the two 10mm nuts that secure the front of the dash to the rain gutter.  This is why you removed the windshield wiper motor… one of them is blocked by the motor.
  9. remove the 10mm nut that secures the rebar to the firewall.  This one is below one of the 10mm nuts that secures the dash by where the windshield wiper motor used to be.


Inside the car:


Remove the dashboard:


  1. remove the steering wheel, ignition switch cover, and both control stalks
  2. (diesel) unscrew the cold start knob bracket and loosen the cable from where it comes thru the dash.  I trimmed the bottom of the hole in the dash to make it easier during reassembly.
  3. remove all dash fascia:  glove box, knee panels, dials, switches, cluster, etc etc etc.  The heater controls do not need to be disconnected.. let ‘em dangle.
  4. remove the plastic emergency brake housing
  5. remove the centre console.. I found I needed to remove the two 13mm nuts that bolt the emergency brake handle down so that I could push the handle out of the way… others seem to know how to wiggle the console out with the handle in place.
  6. disconnect all remaining wiring from behind the dash.  This includes speaker connections, lighting connections to the passenger heater vent, and several grounds
  7. loosen all wiring secured by plastic wiring clamps attached to the dash or heater box.  There are a bunch of them on a row below where the cluster sits, one by the headlight switch, and several on the passenger side.  I found the factory had looped the speaker wiring around the one of the two dash mounting brackets and I had to snip it.  Note the wires zip-tied to the top of the blower fan assembly… snip snip goes the zip-tie.
  8. remove all remaining dash mounting bolts.  These are 8mm bolts: several in the vicinity of  the steering wheel, a couple under the centre console, and several by the passenger side.  There are also one each on the extreme left and right lower sides of the dash, by the door jams.
  9. slowly and carefully, pull the dashboard out of the car… checking for wires that are caught etc. It is not a tight fit and should come out easily… if something binds stop and figure out why !!


Loosen the ReBar:


  1. remove the four big torx bolts that secure the sides of the rebar to the car… they are accessible from the driver and passenger side door jam.
  2. remove the four 13mm nuts securing the reinforcing plate that goes between the centre console hump and the rebar
  3. remove the two 13mm nuts that secure the front of the pedal cluster bracket to the rebar.. this is the only part of the job where you may need to stand on your head to see ‘em initially.
  4. drill out the two shear bolts that attach the steering wheel column to the rebar… trust me, this is quicker than grinding them down or trying to grab them with vicegrips
  5. remove (or snip) the braided copper ground clip on the passenger side of the rebar
  6. working from the passenger seat,  pull out the rebar on that side and lower it to the floor


Loosen the Heater box:


  1. remove the two small black tubes that funnel heat into the centre console channel… one Phillips screw each.
  2. remove the big white plastic plenum at the front of the heater box… two 10mm plastic nuts
  3. remove the smaller black plastic plenum at the front of the heater box… one Philips screw
  4. cut the big plastic ziptie that wraps around the entire heater box assembly on the passenger side


Remove the damn heater core:


  1. (air conditioning) remove the rubber vacuum hose from the heater core
  2. remove the star screw on the center of the dash that keeps the sound damping in place and then pull the sound dampening out of the way… you’re going to need every inch.
  3. using a piece of broomstick, 2x2 etc, carefully pry out the heater-core side of the heater box so you can access and remove  the Philips screw that holds the heater core in place..  Go slow… this is in fact a tight fit.   The plastic vent that blows hot air at the drivers feet will want to bump against the rebar… you may have to move the rebar out a bit further.
  4. FINALLY:  pry further on the heater box with your wooden stick and pull the old heater core straight up and out.  Very very tight fit at this point as the hose connections will just clear the dash and window… nice if you obtain another set of hands for this one final step.
  5. obtain a steam roller and run over the stupid heater core repeatedly while giggling insanely like Chief Inspector Dreyfus from the Pink Panther movies.


Notes on reassembly, which is of course pretty much the reverse of disassembly:


  1. the leaking core probably filled the bottom of the heater box with coolant… I was able to reach in with several rags and mop it up.  Prevents even more greasy steam on your windshield.
  2. good idea to fill the new heater core with water as a test… I’ve seen at least one report of a new core leaking.. man would that suck or what… nobody should ever have to do this job twice ??!!
  3. I had to remove the black plastic cover on the bottom of the new heater core to get it to fit… did a bunch of measuring and convinced myself that I had a slightly wrong part that would never fit down all the way.  Some weather stripping tape was required to get the bottom to seal.
  4. note the two black plastic sealing washers on the two studs that hold the dash to the firewall… according to the Bentley they’re very important and need to be there when you mount the dash back in place.
  5. My hands were too large to fish up and reconnect the speakers, so I pulled the grill covers and speakers and fed the wires up to the top of the dash
  6. Rattling dashboards are a pain.. make sure things go back together tight and secure all wiring within the stock clips or additional zip-ties
  7. I found it helpful to reassemble big items like the heater box, rebar, and dash to the point I could loosely bolt on the firewall bolts and then work inside the car to push things in place, carefully checking for interference, etc.  The final step would be to then tighten the firewall bolts.
  8. I temporarily replaced the two steering wheel shear bolts with normal hexheads but promise yourself to order proper shear bolts from the dealer… according to the Bentley Service Manual they are designed to shear off in the event of an accident... sounds important to me!  I filed flats on both of the new ones before installation so that they can be easily removed the next time... hopefully never.
  9. the rebar has four adjustable spacers that the big torx bolts run thru.. donuts with grooves and a left-hand thread… looks like the factory uses them to get the rebar centred between the door pillars.  One fell out as I loosened the rebar and it took a while to figure out what it was when I found it on the garage floor later.
  10. run the engine for 3-5 minutes with the filler cap off, allow the engine to cool, and then top up the coolant.
  11. on my car the sound damping dropped down a bit after I have everything back together and rubbed against the top of the throttle lever.. causing me some anxious minutes during my test drive until I figured it out.  If I had to do it again I’d use some weather-stripping adhesive or the like to glue the sound damping back in place very securely.
Comments (13)Add Comment
written by Mike Williams, November 19, 2016
After following most of the info in this article I can add a few tidbits. Total time was a bit over 6 hours toke my time and removed the dash completely from the car I wanted as much space as possible. Do not drill or grind out the shear bolts it is unnecessary. If you undo the AC blower box ( I removed mine) and just undo the rebar you’ll have more then enuff room to get the box to where you can pull it out completely or just swap the new HC into place. I did take the time and remove much of the ductwork so nothing got damage and definitely be careful moving the wiring out the way, you can never be too cautious or have too much room.
written by jon, October 20, 2014
any idea if the MK3 write up will mostly work for A4 jetta (2000)? I so want it to be applicable as I haven't found such a good write up for my jetta style.



Hi Jon... I've not done an MK4 heater core (outsourced the last one) but the general steps are the same... steering wheel, cluster, radio etc, then dash. Steering wheel has an airbag which needs to be unlocked from the back, A/C does have to be professionally drained as far as I have been told, and there's at least one hidden bolt thru the firewall that may hold things up a bit.

If you can track down someone with a Bentley Service Manual for the MK4 there's some info in Chapter 80 (Heating and Ventilation).

Good luck!!

written by mike, December 29, 2013
Got my old heater core out of my 94 Jetta and it doesn't have a connection for the vacuum hose any idea what I can do?

Hi Mike... not sure if I totally understand the question... but my understanding is that the vacuum hose is only on cars that were equipped (or set up to be equipped) with air conditioning.

So if you don't have air conditioning no worries.

Or perhaps you mean the core you bought doesn't come with the vacuum hose connection your old core has?

written by Jeff, September 24, 2013
I just replaced the heater core of my 97 Jetta. Vince's excellent post gave me the unreasonably insane confidence to do so. And I can say this is not for the faint of heart, but it is possible. Here are a few points of clarification from my experience.

With regards to parts and tools, there was not enough coolant loss to warrant the elbow by-pass pipe between the hoses. I just let the hoses hang loose with a pan below the car. You will need a 24mm socket to remove the steering wheel nut. A #5 Allen to loosen the air-bag from the steering wheel. I left the air-bag connected, put it atop the steering wheel and once the steering wheel was removed, placed the air-bag back into the steering wheel to set aside. Be careful to make sure the wire coil and a little electrical connecting pin don't fall out of the steering wheel base. The socket required for the 're-bar' dashboard plate is a T-45 Torx.

From inside the engine I located the two screws that hold the heater box, just under the inlet/outlet hoses, through the U-flaps in the firewall foil.

From inside the car I could not easily remove the radio (without security tool) so I left it in the dash and was able to reach around and remove 4 connectors including the coax antenna using some finger gymnastics. After the dash was removed I trimmed out some plastic to make it easier to access those connectors later. Don't be intimidated by the wiring mess. There are only these connectors to disconnect: steering column controls (4 connectors + air-bag) speakers to either side, each with two of the same connectors apiece, the headlight switch cluster, the instrument panel, and radio.

The steering column sheer bolts DON'T need to be removed, although in ignorance I did so without needing to smilies/sad.gif (If, for whatever reason, you need to remove them - the only reasonable way is to grind a slot into the heads with a cut-off wheel. Try to tap them loose with a slot screw-driver and hammer. Then unscrew them using vice-grips clamped to the screw-driver. I replaced them with two 8M x 25mm bolts.) Also, you may find it useful to remove the passenger side air-bag unit to clear the way.

After fighting with the heater box and gaining only one inch of clearance per hour, I finally got TWO eurekas from God! Until that point I had my 4' crow-bar and a saw at the ready - believe me.

ONE: The heater box may be separated into two halves, joined by metal C-clips. Two in front (cabin side). One on top. One at the back (firewall side) and one underneath. I pried them off with a slot screwdriver and subsequently rejoined the box with the two clips on the front and one on top. That made for a more than snug re-join. This meant I did not need to find out what was holding the blower side of the heater to the firewall. I did try but could not determine if it was held by air-conditioner components or what. This was a big help.

TWO: The re-bar dash plate does not need to be lowered to the floor. It needs to be pulled out (toward the passenger) on the passenger side only a few inches. You may find that the torx threaded washer may be jamming the bar. I had to really yank on it to get it free.

Upon removal of the core, I was a little surprised to find only a small amount of corrosion on the bottom of the heater core, but enough to seep toxic fumes. After everything was back together I forgot about adding insulation. I don't think any is required since the old part did not have a plastic bottom cap or any foam. I did have to remove the bottom cap on the new core to fit it. Also my air conditioning requires the vacuum hose connection through the heater core flange. I had to drill through the cut-out on the new heater core flange, pull the old vacuum hose connection out and glue it into the hole I drilled on the new core plastic flange. No big deal.

Having done it this way makes ample clearance for the heater core. (I took some pics.)

Total tools and parts cost for me was about $72. My labor was $15 at 0.50 cents per hour. I live in China. (just kidding)
I spent two and a-half days on the job, including a bicycle run to the store, but the second time - NEVER AGAIN - would be half that with this info.

Enjoy, Jeff
written by Cory Brandt, September 22, 2013
hey, I'm in the middle of doing this job on my car right now. only did heatercores in Neons, a few other Chrysler's, a few GM's.. this one isn't too bad. lots of bolts and screws to mess with, waiting on a torx bit right now to get the knee-bar out. glad I looked this doc. over before i dug into it, thanks for taking the time to write this out. I'm sure i'll be back before the day is over.
written by Bert, September 04, 2012
remove the two 13mm nuts that secure the front of the pedal cluster bracket to the rebar.. this is the only part of the job where you may need to stand on your head to see ‘em initially.

Great instruction!!! Really appreciate it. But can elaborate on the above quoted paragraph? I can't seem to find these nuts, and it seems that these are all that's holding my rebar in now. Thanks again!!

Hi Bert... it's been a while.. but as I recall the pedal cluster nuts are of at the top of the re-bar. If you lie on the floor and look up at the top of the rebar you should see 'em.


written by Lambert J., September 07, 2011
I'm about to attempt this in the coming weekend. However, I'm also replacing the evaporator while I'm in there; It's got a big ole leak and won't hold vacuum. Might as well do it while I got the whole thing apart. Thanks for the great write-up. it's really going to help me. Peace.
written by CHRIS, December 30, 2010
I've done alot of these heater cores over the years and have it down to a science... I can remove/reinstall in just under 2hrs now, used to be about 4hrs in the begining.. lil tip: you don't need to remove the sheer pins from the steering column, just the steering wheel.

On another note.. How do I register for an account on the website, it always says registration closed...

I own a 93 VW TD AAZ


Hi Chris... unfortunately registrations will be remaining closed on for the foreseeable future, until the Administrator/Owner is convinced the board is hacker-proof. It was constantly under attack when registrations were open and it became a full-time job just to keep 'em at bay.

written by Christa , October 30, 2010
We have the car apart and are trying to get the new heater core to fit in the housing, the black footing will fit on the new core but it will not fit into the housing. The old one won't even fit back in with the footing, but if I remove the footing it fits snug as a bug. Can I leave that black plastic footing off?

Hi Christa.... it's hard to know for sure without pictures or having it in front of me, but in general as long as you can convince yourself cold air is not going to leak thru the bottom I'd say go for it.

There are often part revisions over the years so it's possible you have a different model heaterbox, but if you can ensure it seals at the bottom properly (you may need to add some foam, use weather stripping, something like that) it should work. Air leaks are not good: if it leaks at the bottom cold air will be able to bypass the heater core and your car will not be as warm.

If I remember correctly I had to remove the black plastic footing to get my new core to fit... I backed it up with a strip of self-adhesive weatherstripping just be sure... the car was uber-hot in the winter, so it musta worked!

written by jedd, December 14, 2009
very very helpfull! iv looked all over and this is the best write up ive seen. thank you!!
written by Tim, September 15, 2009
Hi Vince,

I have a 99 jetta. While this looks very similar would you know if these steps will work for me. It is the new body style, not the old. I keep hearing from some that you must collect the freon. Others, like yourself, say you don't.
Thanks for any/all help.


Hi Tim:

Sounds like you have an MK3 body style, which is what my instructions were written around. So you should be fine.

If you've got an MK4 chassis I'm not sure... have never done one!

written by Samantha, May 27, 2009
I greatly thank you for this tutoral. I think this is going to sux but I have no choice it has to be done lol. How long does this normaly take to do? Just wondering if I need another car for the week or not smilies/smiley.gif Thank you again, please wish me luck ~samm

Hi Samantha... yeah it's a dirty job but someone has to do it... and it's *way* cheaper if that's you. ;-)

It took me about 8 hours all told...I put on some good music and took my time, labeling as I went etc. It's not difficult.. just time-consuming... and surprisingly satisfying when all is said and done. :-)

Best of luck... let us know how it turns out!

written by jon jepma, January 18, 2009
Thanks for this write up, really helped, smilies/grin.gif, now time to put the dash back in and try it, i hope everything works so i dont have to take it apart again smilies/sad.gif haha. Cheers!

Hi Jon... yeah it's a miserable job... and as you're putting stuff back you pray to whatever gods you can think of "please work, please work".

However, as long as you take it slow and double-check as you go it's not rocket surgery... and think of the $$$$ you're saving doing it yourself!!

Best of luck when you turn the key that first time,


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