P1064 on 2016 Cadillac CTS: Stop/Start System Malfunction Causes and Fixes

What's Unique About the 2016-2016 Cadillac CTS

The 2016 Cadillac CTS, along with its platform-mate the Cadillac ATS, is equipped with the Stop/Start system (RPO KL9) and has a known, widespread issue with the grounding for the system's ultracapacitor. GM released several Technical Service Bulletins, culminating in #PI1519D, which outlines a specific repair for a poor ground connection (G402) that causes code P1064 and a range of related codes. This makes the issue a well-documented pattern failure for this specific platform, often leading to a straightforward fix that doesn't involve replacing expensive components.

Diagnostic Flowchart

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

Symptoms You May Notice

• Check Engine Light is on
• "Stop/Start System Disabled" message on the Driver Information Center (DIC)
• The engine does not automatically shut off when the vehicle comes to a stop
• Engine may not restart after an Auto Stop event, requiring a full key cycle
🎬 Watch: Troubleshooting a Cadillac engine that won't crank due to ground issues.

Most Likely Causes

1. Poor Ground Connection for the Stop/Start Capacitor 🔴 High Probability As documented in GM TSB #PI1519D, the ground stud (G402) for the capacitor can have paint on it from the factory, or the nut can be improperly torqued, leading to a poor connection and high resistance. This is the most common cause for P1064 on the 2016 CTS and ATS.
   How to confirm: Following the procedure in TSB #PI1519D, inspect the G402 ground nut behind the left rear wheelhouse liner. Check for looseness, corrosion, or paint on the contact surface. The nut should be removed and the stud surface cleaned to bare metal.
   Typical fix: Disconnect the main 12V battery negative cable. Safely access the capacitor ground stud by removing the driver's side rear wheel and wheelhouse liner. Remove the 10mm ground nut, clean the contact surface of the stud and the terminal ring to bare metal, and re-torque the nut to specification (9 N·m or 80 lb in).
   Est. part cost: $0-$5 (for cleaning supplies)
2. Weak or Failing 12V AGM Battery 🟡 Medium Probability → Shop Vehicle Battery Stop/Start systems (RPO KL9) are extremely sensitive to battery voltage and health. If the main Absorbent Glass Mat (AGM) battery cannot maintain a stable and sufficient charge (typically above 12V), the system will disable itself as a precaution and may set various fault codes.
   How to confirm: Test the 12V battery with a professional-grade battery tester that can specifically test AGM batteries. The voltage should be above 12.4V with the engine off, and it must pass a load test. A simple voltage check is often insufficient. A GDS2 scan tool may show 'Low Battery' as an 'Autostop Disable Reason'.
   Typical fix: Replace the 12V AGM battery. The new battery must be registered to the vehicle using a capable scan tool to reset the battery monitoring system 🎬 See how to fix and reset the Auto Start-Stop system., which may require a 3-hour sleep cycle to complete its learn procedure.
   Est. part cost: $200-$400
3. Failed Multifunction Energy Storage Capacitor ⚪ Low Probability → Shop Radio Capacitor While less common than the ground issue, the capacitor itself can fail internally over time. It is often misdiagnosed and replaced when the actual fault is the ground connection.
   How to confirm: After confirming the ground connection and main battery are good, this becomes the next likely culprit. Diagnosis involves using a high-level scan tool to command the capacitor to discharge and then monitoring its ability to charge and hold voltage. If it fails these tests, replacement is necessary.
   Typical fix: Replace the Multifunction Energy Storage Capacitor. After replacement, a scan tool may be required to perform a capacitor health reset procedure. The original part number 23176212 has been superseded by 86815665.
   Est. part cost: $150-$300

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner.
2. SAFETY FIRST: Disconnect the negative terminal of the main 12V battery in the trunk. Wait at least 5 minutes for all systems to discharge. TSB PI1519D notes this is critical to prevent accidental discharge of the capacitor when working on the ground.
3. Raise and safely support the rear of the vehicle. Remove the driver's side rear wheel.
4. Remove the rear portion of the wheelhouse liner to access the Multifunction Energy Storage Capacitor and its ground connection.
5. Locate the G402 ground connection for the capacitor. It is a stud with a 10mm nut.
6. Remove the 10mm ground nut. Thoroughly clean any paint or corrosion from the stud's contact surface and the terminal ring using a wire brush or sandpaper to ensure a bare metal connection.
7. Reinstall the ground nut and torque it to 9 N·m (80 lb in).
8. Reassemble the wheelhouse liner and wheel.
9. Reconnect the 12V battery.
10. Clear the trouble codes and start the vehicle. Perform a test drive to see if the Stop/Start system functions and if the code returns.
11. If the code returns, perform a comprehensive load test on the main 12V AGM battery. Replace if it fails the test.
12. If the ground is good and the battery is good, the final step is to diagnose the Multifunction Energy Storage Capacitor itself, likely requiring a scan tool to test its function before deciding to replace it.

Parts You'll Likely Need

• Multifunction Energy Storage Capacitor (OEM #23176212, 86815665 (superseded)) — This is the component that needs to be replaced if it has failed internally, but only after verifying the ground connection and main battery are good. The original part 23176212 is discontinued and replaced by 86815665.
  Trusted brands: ACDelco, Genuine GM
  OEM price range: $200-$300
  Aftermarket price range: $150-$250
• AGM Battery — A weak main battery is a common reason for the Stop/Start system to be disabled, which can sometimes trigger related fault codes. All GM models with Stop/Start use an AGM battery.
  Trusted brands: ACDelco, Bosch, DieHard
  OEM price range: $250-$400
  Aftermarket price range: $200-$300

Related Codes That Often Appear With This One

• P1065 — This and a wide range of codes from P1065 to P108F are part of the same family of faults related to the Stop/Start capacitor and are often caused by the same G402 ground issue, as listed in TSB PI1519D.
• P1070 — Like P1065, this is another code listed in TSB #PI1519D that points to a fault in the Stop/Start capacitor system, frequently triggered by the poor ground connection. On some models, it can also be caused by an ECM software anomaly.

Technical Service Bulletins (TSBs) & Recalls

• PI1519D: Stop/Start System Malfunction or DTC P1064 (and others) Set. This bulletin provides the primary diagnostic path, pointing to a faulty ground connection (G402) as the likely cause. It supersedes previous versions like PI1519C.
• PI1519B, PI1518A, PI1519A: Earlier versions of the TSB, which also link P1064 to the Stop/Start system malfunction. These were superseded by PI1519D.

Platform-Specific Known Issues

• GM Technical Service Bulletin PI1519D specifically calls out a condition where paint on the ground stud (G402) for the Stop/Start capacitor causes a poor connection, leading to code P1064 and others. The fix is to clean the stud to bare metal and properly torque the ground nut.
• This issue is not unique to the CTS; its platform-mate, the 2016 Cadillac ATS, is also explicitly mentioned in TSB PI1519D as being affected by the same G402 ground issue.

Mechanic-Grade Diagnostic Values

• Stop/Start System Inhibit Reasons — expected: All parameters should show 'Not Active' or 'No' for an Auto-Stop to occur.. Failure: A parameter such as 'Low Battery,' 'Engine Temperature,' or 'Brake Booster Vacuum Low 🎬 Watch: How brake assist issues can trigger Auto-Stop system failures.' will be listed as the reason for disabling the system.
• Brake Booster Vacuum — expected: Greater than 45 kPa (7 psi). Failure: A reading less than 45 kPa (7 psi) will inhibit an AutoStop.
• Battery Voltage (Static) — expected: Greater than 12.4V (ideally 12.6V for lead-acid AGM). Failure: A reading below 12V is a direct cause for the Stop/Start system to be disabled.

Scan Tool Commands That Help

• GDS2: ECM/Data Display > Autostop Disable Reason — This is a primary diagnostic step to understand why the Stop/Start system is being inhibited by the ECM. It provides a live list of parameters preventing the system from functioning, which can quickly pinpoint issues like low battery voltage or insufficient engine temperature without guesswork.
• GDS2: Body Control Module > Configuration/Reset Functions > Battery Sensor Module Learn — This function must be performed after replacing the main 12V AGM battery. It resets the battery monitoring system's learned parameters. Failure to perform this reset can lead to improper charging and premature disabling of the Stop/Start system.

Wiring & Ground Locations

• G402 — Behind the driver's side (left) rear wheelhouse liner, near the Stop/Start capacitor. This location is specified in TSB PI1519D for this specific repair.. This is the dedicated ground for the Multifunction Energy Storage Capacitor. As documented by GM, it is the single most common failure point for code P1064 due to paint or improper torque from the factory, causing high resistance.
• GM Ground Naming Convention — Vehicle-wide. GM grounds are numbered based on location: G1xx = Engine bay/front, G2xx = Front passenger cabin, G3xx = Rear passenger cabin, G4xx = Rear of vehicle/trunk area. Knowing G402 is a '400-series' ground confirms its location is in the rear of the vehicle.

OEM Part Supersession History

• 23176212 → 86815665 — The original part was discontinued and replaced by the newer number. Reasons for supersession often include minor design improvements, a change in manufacturer, or consolidation of parts, but no specific reason is publicly documented.