I have a 2003 VW Jetta TDI and the check engine light has been staying on solid (not flashing) for a couple of months. I took it to my local dealer who tested the computer and said that it was the thermostat and a related sensor. That answer didn’t seem to make sense, and unfortunately I didn?t get the code from the dealer at the time. I did though have the code read recently at another shop and it said: “Code P0674 which is #4 glow plug circuit - Q13 - electrical fault sporadic.” Any wisdom out there as to what this means?
General “p” codes will lead you astray when it comes to VW diag. Did they use a generic scan tool or a VW specific scan tool (like VCDS or VAG-COM to oldtimers)? If yes and it’s a glowplug circut fault the first suspects are the glowplugs and glowplug harness. Check the glowplugs, replace if necessary, if the plugs are ok suspect dirt/corrosion in the harness. i have had good success cleaning up high resistance in the gp harness w/some good electrical cleaner like deoxit-5, let it soak into the plug sockets and harness (twist plug sockets upsidedown, let cleaner soak-in). If this works eventually the code will return and the permenent cure is to replace the plug harness. The plug monitoring system on your ALH code TDI engine is very sensitive to small voltage variations and is sometimes prone to false alarms, althought it is the early ALH gp monitor system that is the worst, yours is the latter ALH gp monitor so you may indeed have a bad plug or maybe a false alarm. Do your self a favor and read glowplug 101 at TDIclub.com http://forums.tdiclub.com/showthread.php?t=79819
Thanks for the second re tdiclub Joe, above I pointed him to the specific thread he needs.
Whatever you do OP, DO NOT go back to that dealer that wants to replace a “thermostat or related sensor”. Unless you misunderstood them or the code you have now they have no clue and will cost you needless $'s.
Also do not be afraid to post/ask a ques. or for help at tdiclub AFTER you read up, if you don’t understand, fine but try to read up first. Your problem is most likely a very simple fix. If you post a location you may very well get an offer from a member to meet and get help or a heads up on a competent TDI saavy mechanic/shop. Typically a friendy, helpful bunch, ask for help there if you can’t get through it yourself.
Thanks so much, I will follow your advice and let you know how it turns out 
Good Luck, do let us know what it was and if you are in search of deoxit-d5 try a local music/guitar/sound/stage shop. It’s commonly used in the electronic music/stage world.
Well, how’d you make out?