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AZTechBrian
AZTechBrian, ASE Certified Technician
Category: GMC
Satisfied Customers: 706
Experience:  Owner
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Trouble code p0785 replaced shift silonods works fine cold

Customer Question

Trouble code p0785 replaced shift silonods works fine cold then goes to limp mode when warm. Some say it's the ignition switch other s say it's the computer. Which is it
Submitted: 8 months ago.
Category: GMC
Customer: replied 8 months ago.
No answer yet
Customer: replied 8 months ago.
Did we lose connection
Expert:  AZTechBrian replied 8 months ago.

area all your gauges working correctly?

Customer: replied 8 months ago.
like I said the trouble code p0785 came up for the shift silonods. I replaced them and still did the something. When cold shifts great once it warms up che k engine light comes back on and transmission goes I to limp mode...... I did several reviews from your site and one GMC guy said it was the computer while another said it was the ignition switch... So now I,m lost do t want to keep buying PARTS
Customer: replied 8 months ago.
Every review points to the ignition switch and some point to the computer
Customer: replied 8 months ago.
I have a computer that was out of the same truck do I still need to get it programed if you think it's that
Expert:  AZTechBrian replied 8 months ago.

I have searched through a confirmed fix repair archive for this issue ( not google), and both those items are reported as confirmed fixes for this issue. I assume the other technicians are members of the same technicians network that I am. There is a TSB for this though it specifies issues with the instrument cluster as well as the DTC and staying in 3rd gear, which it sounds like you do have. In the TSB, the reported fix is the ignition switch which is where I would start. Here is the TSB

Electrical - MIL ON/A/T Stuck in 3rd Gear

Bulletin No.: 01-07-30-002C

Date: August 21, 2003

TECHNICAL

Subject: Malfunction Indicator Lamp (MIL) On, Transmission Stuck in Third Gear, Instrument Cluster Inoperative (Replace Ignition Switch)

Models: 1999-2000 Cadillac Escalade 2002-2003 Cadillac Escalade, Escalade EXT 2003 Cadillac ESV 1997-1999 Chevrolet Full-Size Pickup, Suburban, Tahoe (Old Style) 1997-2003 Chevrolet Blazer, Express, S-10 1999-2003 Chevrolet Silverado, Suburban, Tahoe (New Style) 1999-2003 Chevrolet Blazer RH Drive (Export) 2002-2003 Chevrolet Avalanche, TrailBlazer, TrailBlazer EXT 1997-1999 GMC Sierra, Suburban, Yukon (Old Style) 1997-2001 GMC Jimmy 1997-2003 GMC Savana, Sonoma 1999-2003 GMC Sierra, Yukon, Yukon XL (New Style) 2002-2003 GMC Envoy, Envoy XL 1997-2003 Oldsmobile Bravada 2002-2003 Oldsmobile Bravada with 4L60-E or 4L65-E Automatic Transmission (RPOs M30, M32)

Supercede:

This bulletin is being revised to update the model years and to add additional models. Please discard Corporate Bulletin Number 01-07-30-002B (Section 07 - Transmission/Transaxle).

Condition

Some customers may comment on one or more of the following conditions:

^ The Malfunction Indicator Lamp (MIL) is ON.

^ The transmission is stuck in third gear.

^ The instrument cluster is inoperative.

Upon investigation, one or all of the diagnostic trouble codes (DTCs) P0740, P0753, P0758,P0785, P1860 may be set.

Cause

The most likely cause of this condition may be a loss of power to the transmission on circuit 1020. This loss of power may be caused by an open in the ignition switch. This condition may also be the result of an incorrect installation of an aftermarket electronic device such as a remote starter or alarm system.

Correction

With the ignition switch in the RUN position, test for battery voltage at the appropriate fuse shown.

If battery voltage is present at the fuse, inspect circuit 1020 between the fuse and the transmission for possible opens. If battery voltage is not present at the fuse, replace the ignition switch using the Ignition Switch Replacement procedure in the Steering Wheel and Column sub-section of the applicable Service Manual. Refer to group number 2.188 of the parts catalog for part description and usage.

If the vehicle is equipped with an aftermarket electronic device and the DTCs are being set when the device is being used, verify that the appropriate fuse listed above is being supplied battery voltage during operation. If voltage is not present at this fuse, these DTCs will be set due to lack of power at the solenoids. This condition is most likely to occur with an incorrectly installed remote starting system. If this is the case, refer the customer to the installer of the system for corrections.

Expert:  AZTechBrian replied 8 months ago.

Personally I would start with the ignition switch.

Customer: replied 8 months ago.
Put it this way I can hook my computer up to it so when the check engine light comes on a shut it off and every 3 seco DS it comes back on..... So start there. And if that don't work replace the computer
Expert:  AZTechBrian replied 8 months ago.

Start with looking over all wiring and connections at the transmission. Look for damaged wiring, loose connector, etc. If nothing looks suspicious I would address the ignition switch since that does seem to be the more common of the causes.

Customer: replied 8 months ago.
If that don't work I have a computer out of the same year Yukon do I have to have it re programed
Customer: replied 8 months ago.
Is this switch one of GM,s recalls
Expert:  AZTechBrian replied 8 months ago.

You may be OK as long as all the options are the same. If the trucks were identical then you may be fine with an even swap.

Unfortunately I am not seeing this concern under the listed recalls for your truck.

Customer: replied 8 months ago.
Ok I just don't understand why guys on your same network tell me it's the computer and other ones like yourself tell me it's the ignition switch
Customer: replied 8 months ago.
your saying g if I replaced the ignition switch and it doesn't fix the problem I should replace the computer
Expert:  AZTechBrian replied 8 months ago.

I couldn't say for sure what is the cause as there are multiple possibilities that have been verified. If it were in the shop, most guys would address the TSB first and move on from there. I can just say that both the ignition switch and the computer have been confirmed possibilites. I don't think the computer is the most likely though. I think it's most likely the switch or a damaged wire or connection. Again, there is no way for me to tell you with any certainty what the problem is. This is the beauty of this profession ( or lack of) we can spend sometimes days chasing down problems..

Customer: replied 8 months ago.
I hear you you I hate electrical probles
Expert:  AZTechBrian replied 8 months ago.

trust me, we do too. We often lose on them, but that's the nature of the business.

Customer: replied 8 months ago.
I hear you there
Customer: replied 8 months ago.
Ty
Expert:  AZTechBrian replied 8 months ago.

Your welcome.

Expert:  AZTechBrian replied 8 months ago.

If you need further assistance, feel free to hit us up.

Expert:  AZTechBrian replied 8 months ago.

Hi. Just wondering if the TSB was successful in correcting the issue?