U1300 on 2004-2007 Cadillac CTS: Fixing Key Stuck, No-Start, and Communication Faults

What's Unique About the 2004-2007 Cadillac CTS

While U1300 is a generic GM network code, on the 2004-2007 Cadillac CTS it has a very specific and common cause. These vehicles are known for failures within the ignition switch assembly. This failure often disrupts power to essential modules and simultaneously shorts the Class 2 data line, triggering the U1300 code and leading to the widely-reported and distinct symptom of the key becoming physically stuck in the ignition lock cylinder.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Key is stuck in the ignition and cannot be removed.
• Engine will not crank or start (no-start condition).
• Intermittent no-start issues.
• Radio, climate controls (HVAC), and other interior accessories do not power on.
• Check Engine Light is illuminated.
• Multiple warning lights on the instrument cluster.
• Charging system warning or low voltage reading.
• "Service Theft System" message on the Driver Information Center (DIC).

Most Likely Causes

1. Faulty Ignition Switch 🔴 High Probability → Shop Ignition Switch The ignition switch on this platform is a well-documented failure point. Internal contacts wear out, causing a loss of power to multiple circuits, including the key release solenoid 🎬 Watch: A quick sidestep fix to release your stuck key. and modules on the Class 2 data line, triggering the fault.
   How to confirm: With the battery connected, use a multimeter or power probe to check for constant 12V power at the red/white wires on pins 1 and 6 of the ignition switch connector. If power is present going into the switch but accessories and ignition-on circuits are dead when the key is turned, the switch has failed internally.
   Typical fix: Replace the ignition switch assembly. The OEM part number is 84153624, which corresponds to ACDelco D1432D. Common aftermarket alternatives include Standard Motor Products US542.
   Est. part cost: $40-$100
2. Blown Fuse(s) 🟡 Medium Probability An electrical short in a related component or the ignition switch itself can blow the fuse that supplies power to the ignition switch and the key lock solenoid.
   How to confirm: Visually inspect and test fuses. Specifically, check the 10-amp fuse labeled "RIM/IGN SW" in the right rear fuse block (located under the rear seat cushion 🎬 Watch: How to find and replace the RIM/IGN SW fuse. on the passenger side). This fuse powers the Rear Integration Module, Ignition Switch, and Key Lock Cylinder.
   Typical fix: Replace the blown fuse. If the fuse blows again immediately, a short circuit exists and requires further diagnosis.
   Est. part cost: $1-$5

Rare But Worth Checking

• Internal Failure of a Connected Module: A module on the Class 2 network (e.g., BCM, ECM, radio, door module) can short internally, pulling the entire data bus down. Diagnosis involves unplugging modules one by one to see if the short disappears. The factory radio is a known, though less common, module to cause this. This is a time-consuming process.
• Shorted Wiring Harness: The Class 2 data wire (typically a dark green wire on this platform, Circuit 1807) could be chafed and shorting to a metal ground point somewhere in the vehicle. This can be caused by previous repairs, accident damage, or rodent activity.

Diagnosis Steps

1. Verify the primary symptoms: key stuck in ignition and/or no-start condition.
2. Scan all modules for DTCs. Note U1300 and any other 'U' codes indicating loss of communication.
3. Check the battery state of charge and terminal connections. A very low battery can cause numerous electrical issues and should be fully charged before proceeding.
4. Inspect fuses first. Check the 10A fuse labeled "RIM/IGN SW" in the right rear fuse block (under the passenger-side rear seat). Also check the main 60A fuse in the underhood fuse block that supplies power to the rear fuse blocks.
5. If fuses are good, gain access to the ignition switch connector under the steering column by removing the lower dash panels.
6. Using a multimeter, confirm that the two red/white wires (pins 1 and 6) at the connector have constant battery voltage.
7. If power is missing at the connector, trace the wiring back to the fuse block to find the open circuit.
8. If power is present at the connector, the ignition switch is the most likely culprit. Replace the ignition switch.
9. If the issue persists after replacing the switch, advanced diagnosis is needed to find a short on the Class 2 data line. This involves disconnecting modules (Radio, BCM, Door Modules, etc.) from the network one at a time while monitoring the data line voltage to see which module, when unplugged, restores communication.

Parts You'll Likely Need

• Ignition Switch (OEM #84153624) — This is the most common failure point on the 2004-2007 CTS that causes a U1300 code along with a stuck key and no-start condition.
  Trusted brands: ACDelco (OEM - D1432D), Standard Motor Products (US542)
  OEM price range: $65-$120
  Aftermarket price range: $30-$80

Related Codes That Often Appear With This One

• U1000 — A general Class 2 Communication Malfunction code, often stored alongside more specific 'U' codes when the network is down.
• U1040 — Loss of Communication with Electronic Brake Control Module (EBCM).
• U1064 — Loss of Communication with Body Control Module (BCM).
• P1629 — Theft Deterrent System - Cranking Signal Malfunction. All these codes are mentioned in TSB #PIC5460B as being related to the same underlying ignition power issue.

Technical Service Bulletins (TSBs) & Recalls

• PIC5460B: Addresses concerns of the key being stuck in the ignition and multiple DTCs (including U1300) being stored. It applies to both the 2004-2007 CTS and 2004-2009 SRX. It guides technicians to check the Ignition 1 power feed and Class 2 data wire to the ECM.

Platform-Specific Known Issues

• A known issue where the key cannot be removed from the ignition lock cylinder, often accompanied by various communication DTCs including U1300, is documented in Technical Service Bulletin #PIC5460B. The bulletin points to a cause related to a wiring concern in the 'Ignition 1' power feed (Circuit 5290) or the Class 2 data line (Circuit 1807) to the ECM.

Mechanic-Grade Diagnostic Values

• Class 2 Data Bus Voltage (at DLC Pin 2) — expected: Toggling signal between 0V (recessive) and approximately 7V (dominant) when active.. Failure: A steady voltage near 0V indicates a short to ground (U1300). A steady voltage significantly above 0V (e.g., 5V or 12V) indicates a short to power.
• Class 2 Data Bus Resistance to Ground (at DLC Pin 2) — expected: A very high resistance, typically in the mega-ohm range (battery disconnected).. Failure: Anything lower than 350 ohms indicates a short to ground.

Scan Tool Commands That Help

• GM Tech 2: Vehicle DTC Information / DTC Display — To perform a network-wide scan and identify which modules are not responding. If the tool cannot communicate with any module, it confirms the entire data bus is down, pointing to a physical layer fault like a short or open circuit.
• GM Tech 2: Module Information — After establishing communication, this function can be used to see software and calibration part numbers for individual modules, which can be useful for verifying module integrity or checking for required updates.

Wiring & Ground Locations

• SP205 (Splice Pack 205) — Under the driver's side dash, near the steering column. It serves as a central junction point for the Class 2 data lines from multiple modules.. This is the most effective place to isolate individual modules from the data bus for testing. By removing the shorting bar (comb) from SP205, a technician can test each module's data line individually to find which one is shorted to ground.
• Circuit 1807 — This is the GM circuit number for the Class 2 Serial Data line, typically a dark green wire in this vehicle.. This is the specific wire that is shorted to ground when a U1300 code is set. All diagnostic voltage and resistance checks are performed on this circuit.
• G201 — A primary interior ground point, typically located under the dash or in the driver's kick panel area.. A poor ground at G201 can cause issues with the Body Control Module (BCM), which is the Power Mode Master and gateway for the Class 2 network. While not a direct cause of U1300 (a short), a bad ground can cause numerous other communication faults.
• Data Link Connector (DLC) — Under the driver's side of the dashboard, as mandated by OBD-II standards.. Pin 2 of the DLC is the terminal for the Class 2 data line. It is the primary and most accessible point for initial network voltage and resistance tests.

Real Owner Repair Stories

• iATN (International Automotive Technicians Network) (2004 Cadillac CTS 3.6L) — No start, key will not come out of ignition, U1300 code present.
  ❌ Tried (didn't work) Replacing the Engine Control Module (ECM).
  ✅ What actually fixed it The final fix was not posted in the thread, but the story is valuable as it confirms that blindly replacing the ECM is a common and costly misdiagnosis for this exact set of symptoms.
• YouTube - Rico L. (GM Truck (similar Class 2 network)) — No communication with scan tool, multiple warning lights, abnormal voltage on data bus.
  ❌ Tried (didn't work) Initial checks did not immediately reveal the source.
  ✅ What actually fixed it The Electronic Brake Control Module (EBCM / ABS module) had failed internally and was sending over 10 volts onto the 7-volt data line, crashing the network. The fix was unplugging the EBCM to confirm the network came back to life, and then replacing the faulty module.

When the Usual Fixes Don't Work

• While the vast majority of U1300/stuck key/no-start issues on the 2004-2007 CTS are caused by the ignition switch or a fuse, there are documented cases where the fault lies elsewhere. In one instance involving a similar GM Class 2 network, a faulty Electronic Brake Control Module (EBCM) shorted the data line to voltage, causing a total network failure. In another professional forum discussion, a technician replaced the ECM for a U1300 code on a '04 CTS, but it did not fix the problem, indicating the short was in the harness or another module on the bus. These cases highlight the importance of not skipping diagnostic steps and confirming the failure before replacing parts, even when one cause is extremely common.

OEM Part Supersession History

• 12450257 → 23334992, then 84153624 (current) — Standard part evolution for improved reliability or manufacturing changes.
  Heads up: The newest part number, 84153624, is the correct service replacement for all 2004-2007 CTS models.

Model Year Variations Within This Range

• 2004-2007: While engine options changed during this period (notably the V6 and the V8s in the CTS-V), the underlying Class 2 electrical architecture and the components related to the U1300 fault (ignition switch, BCM, splice packs) remained consistent. The diagnostic procedure is the same across this generation.