Reset "Service 4WD" code?

2004 Z71, recently was stuck in 4WD after coming off the beach. I removed the selector switch from the dash and it would work, pop it back in and it wouldn't. Replaced it with a Dorman switch and it now switches fine, but I still get the "Service 4WD" indicator after driving a while. It this a code that need to be reset manually?

2004 Z71, recently was stuck in 4WD after coming off the beach. I removed the selector switch from the dash and it would work, pop it back in and it wouldn't. Replaced it with a Dorman switch and it now switches fine, but I still get the "Service 4WD" indicator after driving a while. It this a code that need to be reset manually?

the 4wd service light comes on when it senses any abnormality.reset is done at each starting of the engine. my guess is the new switch is bad. I would get a GM drive train capable scanner and find out what the code is then post here . the old OEM switch does have solder issues so if you can do it properly re solder the switch pins on that ckt board.

the 4wd service light comes on when it senses any abnormality.reset is done at each starting of the engine. my guess is the new switch is bad. I would get a GM drive train capable scanner and find out what the code is then post here . the old OEM switch does have solder issues so if you can do it properly re solder the switch pins on that ckt board.


I did save the old switch. I''l have a look at it and if I can resolder it, I'll see if that fixes the problem.

Well it seems to have resolved itself. It's gone a few weeks without displaying the service indicator.

Well it seems to have resolved itself. It's gone a few weeks without displaying the service indicator.

probably the switch solder joints . temperature will effect this. also salty air ....

Well, things were fine for a while, then about a month ago, driving in snow, I pushed 4Hi and it just flashed and went back to 2Hi. Repeated it a few times and it engaged. When I got to my destination it wouldn't return to 2Hi. I shut it off, started it, and while it was doing it's startup diagnostic I pressed 2Hi and it returned.

About a month later, I noticed it was a bit noisy on the road, and found that it had somehow jumped into in 4Auto. I had to do the same shut off/start procedure to get it back to 2Hi.

Now it won't light at all when I push 4Hi, 4 Auto, or put in N and push 4Lo. No flash, no light, just stays lit in 2Hi. I had used an inexpensive replacement switch, so I grabbed another at Autozone, but it still behaves the same, and Service 4WD indicator is back.

Guess I'll have to find a scanner that can read DTC's. :banghead:

so the 2wd light is on all the time . will not flash when you push the 4WD button etc.. what / how does the transfer case fluid look like ? level ? have had owners complain about this like you .. they never checked the fluid .. reason was they saw no leaks. then they finally checked the fluid no leak because no fluid LOL... post the drive train GM codes with GM capable reader type .. this will help.

so the 2wd light is on all the time . will not flash when you push the 4WD button etc.. what / how does the transfer case fluid look like ? level ? have had owners complain about this like you .. they never checked the fluid .. reason was they saw no leaks. then they finally checked the fluid no leak because no fluid LOL... post the drive train GM codes with GM capable reader type .. this will help.


Correct on the 2wd light.

I have not checked the fluid. I will do that tomorrow. I'm also planning on cleaning up the ground wire connections, some folks have had success with that as well. My buddy has a scanner, I'll see him over the next day or two and report what we find.


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Well, I replaced the encoder sensor ring, that didn't fix the problem. I just checked the fluid level, it was seeping out the fill hole, so it is full. Color is not blue, just a little bit off clear. Interestingly, as I turned the bolt, I heard a clicking sound that started and stopped a couple of times.

It now wants to jump into Auto4 every time I start it. I have to hold down the 2WD button until well after the startup lights clear before it will settle into 2.

I was able to get the codes scanned and it showed C0306 - Motor A/B Circuit Malfunction, and C0379 - Front Axle Circuit Malfunction.

The history codes showed C0306, C0359, C0374, C0379, C0550, B0790, B2725, U1301. Would be nice if they stored dates with the codes.

I'm thinking replace the encoder motor is the next step? (I've replaced switch, cleaned grounds, replaced encoder sensor)

I never said to replace any encoder ring >>>. the fluid in transfer case must be blue. C306 means the encoder motor is bad. this can be the brake that locks the encoder it can short out or other issue. fluid is clear interesting . perhaps they put some engine oil in it . I would drain out the fluid see what was in there and put in the tracII fluid if it does match your vin#.. this tracII at dealership only so with the vin # you will be sure to have correct fluid. only tighten those transfer case plugs to 15 ft lbs MAX... that is a very soft metal. could be the wrong fluid killed it.

I never said to replace any encoder ring >>>. the fluid in transfer case must be blue. C306 means the encoder motor is bad. this can be the brake that locks the encoder it can short out or other issue. fluid is clear interesting . perhaps they put some engine oil in it . I would drain out the fluid see what was in there and put in the tracII fluid if it does match your vin#.. this tracII at dealership only so with the vin # you will be sure to have correct fluid. only tighten those transfer case plugs to 15 ft lbs MAX... that is a very soft metal. could be the wrong fluid killed it.

Yeah, I saw that some have had luck just replacing the ring, I figured at $30 it was worth a shot. I have seen reports on new Dorman motors going bad in a couple of months, was thinking of going the ACDelco remanufactured route, any thoughts on that?

Yeah, I saw that some have had luck just replacing the ring, I figured at $30 it was worth a shot. I have seen reports on new Dorman motors going bad in a couple of months, was thinking of going the AC Delco re manufactured route, any thoughts on that?

some have purchased on ebay these encoder motors . they payed less than $100.. they did work.

the proper fluid is required because of this design. GM used dexron III in these back in the old days then had to change over to the trac II. on new trucks I believe GM now uses the dexron VI. I would replace the fluid with the proper GM spec . check with dealership as I said before. if it is trac II then you will have to buy at dealership the fluid. if they say dexron VI then you can go at auto store and get it cheaper. if that does not work then replace the encoder . also be careful as those wires are very fragile and must be covered with tape/loom material as many have had these get damaged from tech's being too ruff on handling . GM uses very fragile wiring esp . on the new trucks where wires break for no reason .

some have purchased on ebay these encoder motors . they payed less than $100.. they did work.

the proper fluid is required because of this design. GM used dexron III in these back in the old days then had to change over to the trac II. on new trucks I believe GM now uses the dexron VI. I would replace the fluid with the proper GM spec . check with dealership as I said before. if it is trac II then you will have to buy at dealership the fluid. if they say dexron VI then you can go at auto store and get it cheaper. if that does not work then replace the encoder . also be careful as those wires are very fragile and must be covered with tape/loom material as many have had these get damaged from tech's being too ruff on handling . GM uses very fragile wiring esp . on the new trucks where wires break for no reason .



Thanks JCat. I went with an ACdelco reman, it has a year warranty. I'll replace it and the fluid next week, hopefully this will do it!


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Thanks JCat. I went with an ACdelco reman, it has a year warranty. I'll replace it and the fluid next week, hopefully this will do it!


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post back how it worked out . a re-manufactured encoder I would not buy it. as you know where we live that electrical motor gets messed up with all the road crap and ice melting chemicals .. make sure it is not with any damaged/corroded housing damage.. GM does not use high quality metals on the under body components. my ABS got damaged ECBM because , the screws rusted out then moisture got to the electronic controller.

post back how it worked out . a re-manufactured encoder I would not buy it. as you know where we live that electrical motor gets messed up with all the road crap and ice melting chemicals .. make sure it is not with any damaged/corroded housing damage.. GM does not use high quality metals on the under body components. my ABS got damaged ECBM because , the screws rusted out then moisture got to the electronic controller.

I finally got a chance to install the new encoder motor. The motor seems to be responding to the control button now. I can get it to put the transfer case in Neutral, it goes into 4Lo (I hear the clunk), but the 4Lo light flashes instead of lighting solid, and the service 4WD indicator comes on. Same with 4Hi and 4Auto, the lights continuously flash instead of lighting solid.

I'm going to stop by my buddy's place and clear the codes to see if it throws any new codes. I'm also wondering if the existing codes can be confusing the TCCM?

I finally got a chance to install the new encoder motor. The motor seems to be responding to the control button now. I can get it to put the transfer case in Neutral, it goes into 4Lo (I hear the clunk), but the 4Lo light flashes instead of lighting solid, and the service 4WD indicator comes on. Same with 4Hi and 4Auto, the lights continuously flash instead of lighting solid.

I'm going to stop by my buddy's place and clear the codes to see if it throws any new codes. I'm also wondering if the existing codes can be confusing the TCCM?

with what you posted at the start I thought this maybe be multiple problems. as I said the code you gave , points to encoder motor N/G... so it now does move . next step is get the drive train codes post here and perhaps we can narrow it down.

the drive train codes reset on each start. the light flashing means the feedback is wrong so need code ..

what fluid did the transfer case have in it ? what did you put in it ?

plugs are 15 ft lbs max torque no more..

with what you posted at the start I thought this maybe be multiple problems. as I said the code you gave , points to encoder motor N/G... so it now does move . next step is get the drive train codes post here and perhaps we can narrow it down.

the drive train codes reset on each start. the light flashing means the feedback is wrong so need code ..

what fluid did the transfer case have in it ? what did you put in it ?

plugs are 15 ft lbs max torque no more..

I haven't changed the fluid yet, had to do the motor by crawling under the truck. I'll get the codes read tomorrow.

The code is C0379 - Front Axle Circuit. I guess it's time to replace the actuator?

The code is C0379 - Front Axle Circuit. I guess it's time to replace the actuator?

the c0379 does point to the front diff locking actuator. what some have found is metal chunks interfering with the actuator operation. If you do use your truck down on the beach often as I do know friends that do this every nice weekend , the drive train does work extra hard in the deep soft sand , so you best do more fluid checking and replacing to avoid this type stuff..

the c0379 does point to the front diff locking actuator. what some have found is metal chunks interfering with the actuator operation. If you do use your truck down on the beach often as I do know friends that do this every nice weekend , the drive train does work extra hard in the deep soft sand , so you best do more fluid checking and replacing to avoid this type stuff..

Yes, this truck is on Duxbury beach every summer. I'll replace the actuator and see what happens. And I'll put fluid replacement on the calendar too. Thanks!

Any suggestions on removing a stuck actuator? It won't budge with vice grips on it. Can I heat the housing where the threads are? I'm just worried about heating the plastic nut surface.


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Tried heating it, no dice. Will have to wait till I can get it on a lift next week. Probably a good thing, I'm about to lose my mind. Also noticed Diff fluid is low, seals are wet. Other than that, life is good... :banghead:


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So I was able to get the old actuator out on a lift - the housing threads has corrosion in them so it was a b!tch to get the old one out, also had teflon tape on it.

I was able to stick my finger in the hole, press the actuator, and spun a tire to see the front shaft spinning, so it would seem that part is able to be engaged if the actuator is powered.

Installed the new actuator, changed the fluid, buttoned it back up, and it's still throwing the same C0379 code.

Is there an easy way to test the harness?

somehow I missed your other posts. the locking actuator replaced could be the wiring also when you changed the diff oil any metal ? you sure the teeth are good no debris interfering. put the old actuator to the connector , and see if it moves. if the actuators move it maybe the diff...

somehow I missed your other posts. the locking actuator replaced could be the wiring also when you changed the diff oil any metal ? you sure the teeth are good no debris interfering. put the old actuator to the connector , and see if it moves. if the actuators move it maybe the diff...

There was a tiny bit of metal on the screw head, nothing out of the ordinary according to my mechanic.

I'll try the old actuator to see what it does, also check the voltage on the plug.

There was a tiny bit of metal on the screw head, nothing out of the ordinary according to my mechanic.

I'll try the old actuator to see what it does, also check the voltage on the plug.

this code throws when the locking actuator does not engage in 10 sec or less... so very possible something interfering .. how did the gear oil look ? wiring connector could be corroded ..see if pins green ..

this code throws when the locking actuator does not engage in 10 sec or less... so very possible something interfering .. how did the gear oil look ? wiring connector could be corroded ..see if pins green ..

Gear oil was ugly black.:rolleyes:

Gear oil was ugly black.:rolleyes:

the black means it was running very hot .. this can cause the gear teeth to crack/break.

when you fix this check the diff fluids because on the beach the diff''s work very much harder than in snow.. also drop drive shaft and grease the yoke with moly EP2 grease .

Jcat, I was able to get a TCCM from an 03 that was parted out at a body shop. Do they need reprogramming or can I just slap it in? (I still have to check the actuator harness but I'm thinking ahead)


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I said before connect the actuator up with it out of the diff , see if it operates.

check connectors for any green or other corrosion . TCCM rarely fails and I am not sure but others have had to get it programmed .I doubt that is the problem because the codes say no.

So I connected the old actuator and it did not operate, which led me to start testing the harness. The ground did not show continuity, so I went back to check the connections that I had cleaned a couple months back. Upon closer inspection, the ground from the actuator was broken, but since it was taped to another larger ground and crimped in the same connector, it had looked fine, until I pushed on it with a screwdriver. Then it looked like this:

http://img.photobucket.com/albums/v293/yakkk/IMG_8071_zpsicmjre0v.jpg

I added another connector and tightened it down, and lo and behold, I have 4WD!

What a freakin ordeal, I definitely learned a lesson here. Thanks for all your help!

Great lesson. Never overlook the simple things.

Well good that you reported the fix to this problem . wiring issues are time consuming and if YOU did not fix it , and they guess on changing all the parts first , it would cost $$$... posting the fix will help others . not surprised the 4wd computer was not the problem as the code DID reveal that locking actuator was NOT working .. wiring problems on the encoder is quite common as those wires are very easy to break another very common failure .. The weather looks good for the beach now your ready to enjoy .. I am getting my boat ready for next weekend now that the rain has stopped.. Good Luck..

If anyone needs a selector switch, hit me up, I've got a spare.


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So I connected the old actuator and it did not operate, which led me to start testing the harness. The ground did not show continuity, so I went back to check the connections that I had cleaned a couple months back. Upon closer inspection, the ground from the actuator was broken, but since it was taped to another larger ground and crimped in the same connector, it had looked fine, until I pushed on it with a screwdriver. Then it looked like this:

http://img.photobucket.com/albums/v293/yakkk/IMG_8071_zpsicmjre0v.jpg

I added another connector and tightened it down, and lo and behold, I have 4WD!

What a freakin ordeal, I definitely learned a lesson here. Thanks for all your help!

I always find it funny how this ground causes so many problems, but yet its the common ground for a lot of things, depending on year. some only have 1 or 2 wires to one eyelet while others have many wires and eyelets. I redo a lot of these grounds, because the rust is so bad here, they rot off, but what I said about the problems, sometimes its 4x4, mostly abs, sometimes cruise, some times fuel injectors, but the stupid part it only seems to effect one at a time, even though they are one a common ground that is rotted off.