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Discussion Starter · #1 ·
Thank you for add btw...

Trying to help an elderly relative fix their cts, only has about 24,000 miles on it... has always given them issues since they bought the car used.. explains the super low miles i guess... anyways, after hunting down a parasitic draw... fixed by pulling the audio and amp fuses, and iirc the moon roof fuse.. even though there is no moon roof, brought amp draw down to good levels.. bought a new battery for good messure.. good voltage.. still wont start via ignition and key... but will start with key in on position and jump the large purple wire to the nut in the under hood fuse box.. starts right up

Being it starts just fine when the wires are shorted, does this still show signs of failing ecu or a safety or security feature like brakes etc? Does this bypass these systems perhaps?

After i short the wires, let it run, turn car off, lock doors, let it sit for a few minutes, etc, unlock, and can start car like normal.. wth..

Also... had error codes P0700 and u0100.. which after researching seemed to be related to bad fuses and bad battery.. well after pulling said fuses and getting new bat, and clearing codes, the codes came right back. ..

Any help would be much appreciated.

Thank you.
 

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'93 SedanDeville 60 Special
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623 Posts
Hard to answer since you did not state what year this CTS is ?

P0700 is set when the PCM detects a malfunction in the transmission control system.
This malfunction causes the vehicle to set a Check Engine Light and trigger a failsafe mode.
Once the failsafe mode is set it will remain until the fault is repaired or normal operation is detected.

Could be a wiring problem between the PCM and the TCM of the tranny

Or
Some of the most common causes of transmission codes include, but are not limited to:
  • Low or dirty transmission fluid
  • Faulty speed sensors
  • Wiring harness issues, such as random open circuits
  • Malfunctioning shift solenoid
  • Valve body irregularities
  • Converter clutch
In rare cases, a faulty transmission control module maybe at fault
The U code is a comm issue, it could be on random basis the PCM and TCM cannot exchange data.
I'd look for a wiring problem first
 

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'93 SedanDeville 60 Special
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Try cleaning the pins of the wiring connectors of the TCM first and if not the problem also clean the wiring pins from TCM to the PCM
Look for any of those wires cut, burnt, etc, make sure the legs of the fuse(s) for TCM are clean also
You can get a MAF cleaner spray almost anywhere which is a electronics cleaner

Ask when was the last time or mileage the tranny fluid was flushed and replaced
 

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Discussion Starter · #5 ·
trans fluid: Digi display says 75% life. I have a feeling it has never been serviced during their ownership

After doing more digging.. i noticed the TC light stays on, and there is no check engine light when key is switched to ON.. to my understanding is an indication of a bad ECM..

At this point i think we may be looking at ordering one, if they decide to spend the ~$360 for it from flagship..
 

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'93 SedanDeville 60 Special
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I assume you mean ign key ON, engine is OFF ?

If so it could be the CEL bulb itself is bad or a wiring issue,
Or someone in past did not want CEL light on for like smog testing and did something to prevent it going ON
Could on purpose is cause CEL to me on like disconnect the MAF sensor and see if bulb lights
or using a OBD-II scanner is look at the CEL PID and see if PCM commands CEL on or not
Or if scanner supports bi-directional is send command to turn CEL ON

Before spending money on a new PCM is to measure at the PCM for the wiring pins for the CEL and see if a issue with PCM or not
TC on could be the ETBCM or another wiring issue
 

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2005 CTS 3.6L V6
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trans fluid: Digi display says 75% life. I have a feeling it has never been serviced during their ownership

After doing more digging.. i noticed the TC light stays on, and there is no check engine light when key is switched to ON.. to my understanding is an indication of a bad ECM..

At this point i think we may be looking at ordering one, if they decide to spend the ~$360 for it from flagship..
I agree with Team ZR-1, bad wiring will cause all sorts of issues so it’d be best to eliminate that first.

You may be on to something with the CEL though, mine is definitely good but went out after an ECM failed programming, but it never showed trans fluid or engine oil life. If it turns out to be the bad checkout ASAP Speedo, they’ll fix yours and send it back for $285.
 
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