P2535 on 2020-2023 GMC Sierra 2500: Ignition Switch Circuit High Causes and Fixes

What's Unique About the 2020-2023 Gmc SIERRA 2500

For the 2020-2023 GMC Sierra, the P2535 code is frequently linked to specific electronic module issues rather than just a simple mechanical ignition switch failure. Several Technical Service Bulletins (TSBs) point towards the Body Control Module (BCM) failing to de-energize the ignition circuit after shutdown, leading to a dead battery and setting this code. TSB PIT5817A pinpoints this failure to the BCM's internal 'ignition 1 relay (KR73) driver' remaining energized, which keeps power on circuit 5199 after the ignition is off. Another TSB notes that P2535 can appear after replacing the fuel pump driver module, indicating a potential software or communication issue between modules.

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 (SES/MIL) is on
• Engine will not crank or has a no-start condition
• Intermittent stalling, sometimes after a remote start attempt
• Dead battery, as modules may stay powered on after the ignition is off
• Service 4WD message may appear
• Relay clicking sound from the fuse box
• Odd electrical behavior, like the radio or dash lights acting up, flickering, or staying on
• Reduced Engine Power message followed by other warnings like 'Service Parking Brake'

Most Likely Causes

1. Faulty Body Control Module (BCM) 🔴 High Probability → Shop Body Control Module A known issue documented in TSB #PIT5817A where the BCM's ignition relay driver (for relay KR73) remains energized after shutdown, keeping power on the ignition circuit 5199 and causing a parasitic drain.
   How to confirm: A technician would follow diagnostic procedures for P2535. The key step is to test for voltage on circuit 5199 at the BCM with the ignition turned off. If voltage is present, the BCM is faulty and needs replacement.
   Typical fix: Replacement of the Body Control Module (BCM) and subsequent programming by a dealer or qualified shop with access to GM's software. This is not a plug-and-play part. 🎬 Watch: Step-by-step guide on how to replace a BCM
   Est. part cost: $300-$600
2. Faulty Ignition Switch 🟡 Medium Probability → Shop Ignition Switch This is a common cause across many vehicles. Internal contacts can wear out or short, sending an incorrect voltage signal.
   How to confirm: Test the voltage at the ignition switch's output connector in each key position (Off, Accessory, Run, Start). The voltage should match the manufacturer's specifications.
   Typical fix: Replace the ignition switch.
   Est. part cost: $50-$150
3. Wiring Harness Issue (Short to Power) 🟡 Medium Probability Wiring can become damaged or corroded. A well-documented issue on this platform is engine harness chafing (per TSB 21-NA-149) at points like the ECM/TCM bracket, driver's side upper control arm, and shock tower bolt, which can cause shorts. Also, check for damage under the driver's sill plate (circuit 5199) per TSB PIT5391E. Improperly installed aftermarket accessories can also tap into and short the ignition circuit.
   How to confirm: Visually inspect the engine wiring harness at the known chafe points for rubbing or exposed wires. Also inspect wiring under the driver's sill plate. Use a multimeter to check for a short to voltage on the signal wire.
   Typical fix: Repair the damaged section of the wiring harness with solder and heat shrink, then protect it with abrasive-resistant tape and secure it away from sharp edges.
   Est. part cost: $10-$100

Rare But Worth Checking

• Water Intrusion in Headlamp Assembly: → Shop Headlight Assembly TSB #PIT5546G notes that water getting into a headlamp assembly or its connectors (X110, X120) can back-feed voltage into switched ignition circuits (like fuses F26UA, F31UA, F34UA). This keeps modules awake, drains the battery, and can trigger P2535.
• Software Issue After Module Replacement: TSB #PIT6396A explicitly states that P2535 can be set after replacing the fuel pump driver module. The interim fix from GM involves installing a diode in line on the water-in-fuel sensor circuit (circuit 6862) to correct the issue.

Diagnosis Steps

1. Scan the vehicle for all stored DTCs in all modules (ECM, TCM, BCM, IPC). Note any other codes that are present.
2. Check for relevant Technical Service Bulletins (TSBs). TSBs PIT5817A, PIT6396A, and PIT5546G are highly relevant.
3. With the ignition off for several minutes, check for voltage on switched ignition fuses in the underhood fuse block (e.g., F26UA, F31UA, F34UA). There should be 0 volts. If voltage is present, it indicates a module is not powering down, potentially due to headlamp water intrusion or a faulty BCM.
4. If a dead battery is the primary symptom, follow the procedure in TSB PIT5817A. With the ignition off, test for voltage on circuit 5199 at the BCM. If voltage is present, the BCM is the confirmed cause.
5. Visually inspect the engine wiring harness for chafing, especially at the ECM/TCM bracket, driver's side upper control arm, and shock tower bolt, as detailed in TSB 21-NA-149.
6. Inspect wiring under the driver's sill plate for corrosion or damage, a known issue area.
7. If a recent repair was performed (like replacing the fuel pump driver module), consult TSB PIT6396A and be aware that a specific wiring modification (diode installation) may be required.
8. If no TSBs apply, test the ignition switch itself. Using a multimeter, check the voltage on the output wires in each key position to ensure they match specifications.

Parts You'll Likely Need

• Body Control Module (BCM) (OEM #Varies by VIN and options. Must be matched exactly.) — A documented common failure point on these trucks where the BCM fails to shut down the ignition circuit (specifically circuit 5199), causing P2535 and a dead battery, per TSB PIT5817A.
  Trusted brands: ACDelco (Genuine GM)
  OEM price range: $300-$600
  Aftermarket price range: $200-$450 (Used/Remanufactured, but requires programming)
• Ignition Switch — A standard potential cause for this code if the internal contacts fail and send a high voltage signal. Less common than the BCM on this platform, but still a possibility.
  Trusted brands: ACDelco, Standard Motor Products
  OEM price range: $70-$150
  Aftermarket price range: $40-$90

Related Codes That Often Appear With This One

• P0700 — This code indicates a request from the Transmission Control Module (TCM) to illuminate the check engine light. It often appears with P2535 because the TCM also relies on the ignition voltage signal to function correctly, and TSB PIT5817A lists it as a common companion code.
🎬 See how to fix P0700 and related electrical codes
• P12A6, P10DA, C111B — These codes may appear alongside P2535 specifically after a fuel pump driver module replacement, as documented in TSB #PIT6396A.
• U300D — This Instrument Panel Cluster (IPC) code is also listed alongside P2535 in TSB PIT5817A, related to the BCM failing to power down correctly.

Technical Service Bulletins (TSBs) & Recalls

• PIT5817A: Addresses a dead battery and DTC P2535 caused by the BCM keeping the ignition circuit (5199) powered on after shutdown.
• PIT6396A: Communicates an issue with DTC P2535 setting after the replacement of the fuel pump driver module, requiring a diode to be installed on circuit 6862 as an interim fix.
• PIT5546G: Provides a diagnostic path where water intrusion into headlamps or connectors X110/X120 can back-feed voltage onto ignition fuses, triggering P2535 and draining the battery.
• PIT5391E: Discusses loss of Ignition 1 voltage on slightly older models, pointing to potential terminal issues at the fuse block and wiring damage under the d

Platform-Specific Known Issues

• TSB PIT5817A points to the Body Control Module (BCM) as a primary cause, where its internal ignition 1 relay (KR73) driver remains active after shutdown. This keeps voltage on circuit 5199, leading to P2535 and a dead battery.
• TSB PIT6396A indicates that DTC P2535 can be set after replacing the fuel pump driver module, suggesting a software or configuration issue that GM is aware of. The fix involves installing a diode on a specific circuit.
• TSB 21-NA-149 (and others) document a widespread issue with engine wiring harness chafing on the T1 platform, which can cause a variety of electrical faults and DTCs, including potential no-start conditions that could be related to P2535.

Mechanic-Grade Diagnostic Values

• Voltage on switched ignition fuses (e.g., F26UA, F31UA, F34UA, F40UA) with ignition OFF. — expected: 0 Volts. Failure: Any voltage present indicates a module is not sleeping or there is a short/back-feed.
• Voltage on ignition switch output circuit (e.g., Circuit 5199) with key in RUN/START position. — expected: Approximately 9V - 14.5V (near battery voltage). Failure: Voltage significantly higher than battery voltage (e.g., >16V) or voltage present when key is OFF.
• Resistance of a chassis ground point for the ignition circuit to the negative battery terminal. — expected: ≤0.1 Ω. Failure: A higher resistance value indicates a poor ground connection, which can cause voltage irregularities.

Scan Tool Commands That Help

• GDS2 (or equivalent professional scanner): BCM Programming/Configuration — Required after replacing the Body Control Module. The new module must be programmed with the vehicle's specific configuration (As-Built data) to function. The vehicle will not start until this is done.
• GDS2 (or equivalent professional scanner): 30-Minute Security Relearn (Passlock) — After a new BCM is programmed, this procedure must be performed to synchronize the vehicle's anti-theft system with the ignition keys. It involves cycling the key and waiting for specific intervals.
• GDS2 (or equivalent professional scanner): Setup SDM (Sensing and Diagnostic Module) — If an airbag warning light is present after BCM replacement with DTC B1001, this function is used to configure the new BCM with the airbag system.
• GDS2 (or equivalent professional scanner): Brake Pedal Position Sensor Learn — On some GM vehicles, the brake lights may operate incorrectly after BCM replacement. This calibration function, found in the BCM configuration menu, relearns the BPP sensor's position.

Wiring & Ground Locations

• BCM (Body Control Module) — Under the driver's side (LH) of the dashboard, near the steering column.. The BCM is the primary controller for the ignition circuit and a common failure point for P2535 per TSB PIT5817A.
• Circuit 5199 — This is the Ignition 1 voltage circuit. A known problem area is under the driver's sill plate, where it can become damaged or corroded.. TSBs specifically call out inspecting this circuit for high resistance or shorts when diagnosing P2535 and related no-start/stalling issues.
• KR73 Ignition Main Relay — There are two such relays. One is in the Underhood Fuse Block (X50A) and the other is in the Left I/P (Instrument Panel) Fuse Block (X51L).. These relays supply power to the ignition circuits. A stuck or failing relay can cause voltage issues, although the BCM driver controlling the relay is a more common fault.
• Connectors X110 & X120 — These are major electrical pass-through connectors related to the front lighting and body harnesses.. TSB PIT5546G identifies these connectors as potential points for water intrusion, which can back-feed voltage onto ignition circuits and trigger P2535.
• Fuse F40UA — A 20-amp yellow fuse located in the Underhood Fuse Block (X50A).. This fuse powers the Seat Heating Control Module. A short in that module can back-feed voltage onto the main ignition bus (KR73), causing a parasitic draw and P2535. Removing this fuse is a quick diagnostic step.

"I Checked Everything" — The Actual Cause

• A technician may perform all standard electrical checks on the ignition switch and BCM circuits and find no faults, but the actual cause could be water intrusion in a headlamp assembly or its connector (X110/X120), which back-feeds voltage onto switched ignition fuses, keeping modules awake and setting P2535. This is documented in TSB PIT5546G.
• The vehicle may set a P2535 after a seemingly unrelated repair, such as replacing the fuel pump driver module. Standard diagnostics for the ignition circuit will lead nowhere. The actual cause is a software/logic issue that requires a specific wiring modification (installing a diode on the water-in-fuel sensor circuit) as an interim fix per TSB PIT6396A.

When the Usual Fixes Don't Work

• While the most common cause for P2535 on this truck is a faulty BCM, simply replacing the BCM may not fix the issue. TSBs provide strong counter-evidence to this being the only solution. For example, if the code appeared after a fuel pump driver module was replaced, the true fix is a wiring modification with a diode, not another new BCM. Similarly, if the issue is intermittent and occurs in wet weather, the root cause could be water in a headlamp connector back-feeding voltage, as noted in TSB PIT5546G. In these cases, replacing the BCM would be an expensive misdiagnosis.

OEM Part Supersession History

• 13520409 → 13529041 — Part revision or manufacturer change.
  Heads up: Part 13534086 is also listed as an interchange for 13529041. Due to multiple overlapping part numbers (including 13536089 and 13535137), the correct BCM must be selected by VIN. All replacements, new or used, require programming to the specific vehicle.

Model Year Variations Within This Range

• 2020-2022 (Pre-Refresh) vs. 2022.5-2023 (Post-Refresh): The 2022 mid-cycle refresh introduced the 'Global B' electrical architecture, replacing the 'Global A' architecture of earlier models. This fundamentally changes module communication and security. While the root causes of P2535 may be similar, the BCM part numbers, software, and programming procedures are different between the two architectures. A technician must use the correct software for the vehicle's specific platform.