P2537 on 2014-2018 GMC Sierra 1500: Ignition Circuit Low Voltage Causes and Fixes

What's Unique About the 2014-2018 Gmc SIERRA 1500

For the 2014-2018 Sierra and its platform mates, this code is specifically addressed in GM Technical Service Bulletins. These bulletins point to issues with the 'accessory wakeup' circuit, designated as Circuit 5985 (a violet wire). This circuit is known to develop high resistance or an open (break) in the wiring harness, particularly in the section that runs under the driver's side underhood fuse block 🎬 Watch: How to replace the engine wiring harness junction block. or along the left frame rail. This failure prevents the ECM and TCM from 'waking up' properly, leading to unique symptoms like the blower motor staying on after the truck is shut off or the remote start being inoperative.

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
• Blower motor for climate control stays on after the engine is turned off
• Remote start does not work
• Power liftgate (if equipped) is inoperative when the ignition is off
• Accessories like the radio or power windows may not work correctly
• Potential no-start or intermittent starting issues
• Crank but no start, or starts and then stalls

Most Likely Causes

1. Wiring Fault in Accessory Wakeup Circuit (5985) 🔴 High Probability This specific circuit is highlighted in manufacturer TSBs (PIT5308D), indicating it's a known failure point. The violet wire is prone to chafing and breaking in the harness under the driver's side underhood fuse block or along the frame rail.
   How to confirm: Using a voltmeter, check for battery voltage on the violet wire (Circuit 5985) at the ECM connector X1, pin 43 and the TCM connector X1, pin 48. If voltage is low or absent with the ignition on, inspect the wiring harness between the BCM, ECM, and TCM for damage, paying close attention to the area under the main fuse block in the engine bay.
   Typical fix: Repair the damaged section of the violet wire. This usually involves locating the break, splicing in a new piece of wire, and ensuring a solid, heat-shrunk, and insulated connection.
   Est. part cost: $5-$25
2. Faulty Ignition Switch 🟡 Medium Probability → Shop Ignition Switch The internal contacts of the ignition switch can wear out over time, leading to a voltage drop when in the accessory or run position.
   How to confirm: Test the voltage at the ignition switch's accessory output terminal. If the voltage is low directly at the switch but the input voltage is good, the switch itself is likely faulty.
   Typical fix: Replace the ignition switch. This requires removing the steering column shrouds.
   Est. part cost: $40-$100
3. Blown Fuse ⚪ Low Probability While less common, a fuse related to the BCM or ignition circuits could be the cause. A short circuit elsewhere could cause the fuse to blow.
   How to confirm: Visually inspect and test fuses related to the ignition switch and Body Control Module. Check the fuse boxes in the engine bay and on the sides of the dashboard.
   Typical fix: Replace the blown fuse. If it blows again, further diagnosis is needed to find the short circuit.
   Est. part cost: $1-$5

Rare But Worth Checking

• Faulty Body Control Module (BCM): → Shop Body Control Module The BCM is the source of the accessory wakeup signal. If the wiring and ignition switch are confirmed to be good, the BCM's internal driver for this circuit may have failed. This is an uncommon and more expensive repair that requires programming.
• Poor Ground Connection: A loose or corroded ground connection for the BCM or related components can cause a variety of electrical issues, including this code. Key grounds to inspect on the K2XX platform include those on the A-pillar and under the radiator support.

Diagnosis Steps

1. Scan the vehicle for all DTCs. Note if P2537 is present with communication codes like U0100 or U0101.
2. Check the battery voltage to ensure it is at a healthy level (above 12.4V). A low battery can cause various voltage-related codes.
3. Inspect fuses related to the ignition, BCM, and accessory circuits. Check the fuse panels on both sides of the dashboard and under the hood.
4. If symptoms from TSB #PIT5308D are present (blower stays on, remote start inop), prioritize checking Circuit 5985.
5. Using a wiring diagram, locate the violet wire for Circuit 5985. Key test points are ECM connector X1, pin 43 and TCM connector X1, pin 48. The source is BCM connector X6, pin 1.
6. With the ignition in the 'Run' position, use a voltmeter to measure the voltage on Circuit 5985 at the ECM or TCM. It should be close to battery voltage (approx. 12V).
7. If voltage is low or zero, inspect the wiring harness for any visible damage, corrosion, or loose connections, focusing on the area under the driver's side underhood fuse block and along the left frame rail.
8. TSB PIT5308D also recommends a load test: connect a small 194 bulb between Circuit 5985 and a good ground. When the BCM is awake (e.g., ignition on), the bulb should light brightly. If it's dim or doesn't light, the wire has high resistance or is open.
9. If voltage on Circuit 5985 is good, test the ignition switch output. Check for proper voltage at the switch's accessory terminal when the key is in the accessory/run position.
10. If the ignition switch and wiring test good, the problem may be with the Body Control Module (BCM) itself, which would require more advanced diagnostics.

Parts You'll Likely Need

• Wiring Repair Supplies — The most frequent fix is a simple wire repair on the known problematic Circuit 5985, requiring only basic supplies like wire, butt connectors, and heat shrink tubing.
  Trusted brands: 3M
  OEM price range: $5-$25
  Aftermarket price range: $5-$25
• Ignition Switch (OEM #23377828) — If the internal contacts fail, the switch will not provide the correct voltage to the accessory circuit, triggering the code. This is a less common but possible cause.
  Trusted brands: ACDelco (GM OE# 23377828, Aftermarket# D1491F), Standard Motor Products
  OEM price range: $60-$120
  Aftermarket price range: $40-$80

Related Codes That Often Appear With This One

• P129D — This code is also mentioned in TSB #PIT5308D alongside P2537, relating to issues with the blower motor staying on, particularly in diesel models, but indicates a related control module communication problem.
• U0100 — Lost Communication With ECM/PCM 'A'. This can be set concurrently if the ECM fails to 'wake up' due to the lack of signal on Circuit 5985.
• U0101 — Lost Communication with TCM. This can be set concurrently if the TCM fails to 'wake up' due to the lack of signal on Circuit 5985.

Technical Service Bulletins (TSBs) & Recalls

• PIT5308D: Addresses symptoms like the blower motor staying on or remote start being inoperative, linking them to DTC P2537 and a fault in the accessory wakeup circuit (5985). It provides specific diagnostic steps, including checking voltage at the ECM/TCM and identifying common chafe points in the harness.

Platform-Specific Known Issues

• Common Break in Circuit 5985: A known issue documented in TSB #PIT5308D involves a fault in the accessory wakeup signal (Circuit 5985). Owners and technicians frequently find the violet wire for this circuit is broken inside the main wiring harness located under the under-hood fuse block on the driver's side of the engine bay. The harness can rub or get stressed in this area, causing the wire to fatigue and break over time, leading to the P2537 code and related symptoms.
• Real-World Repair Story: A user on the gm-trucks.com forum with a 2014 Sierra experienced the classic symptoms: blower motor staying on and code P2537. Following advice, they located the large wiring harness under the driver-side fuse box. After removing the electrical tape and conduit, they found a broken purple wire (Circuit 5985). They repaired the wire using a butt connector and heat shrink, which completely resolved the issue. This experience is a textbook example of the most common failure mode for this code on this platform.

Mechanic-Grade Diagnostic Values

• Accessory Wakeup Circuit (5985) Voltage — expected: Approximately 12V (Battery Voltage) when the BCM is awake (e.g., ignition ON).. Failure: The TCM will set DTC P2537 if it detects less than 6 volts for 1 minute. A reading below 9V generally indicates a problem.
• Accessory Wakeup Circuit (5985) Load Test — expected: A 194-style bulb connected between Circuit 5985 and a good ground should illuminate brightly when the BCM is awake.. Failure: If the bulb does not light or is dim, it indicates an open circuit or high resistance. Voltage measured across the lit bulb should be at least 11 volts.
• Accessory Wakeup Circuit (5985) Continuity — expected: Less than 2 ohms of resistance from end to end.. Failure: 2 ohms or greater indicates high resistance or an open in the circuit that needs repair.

Scan Tool Commands That Help

• GDS2 (or equivalent professional scanner): BCM Data Display: 'Ignition Accessory/Run' parameter — Use this to verify if the BCM itself knows that the ignition is in the Accessory or Run position. With the key in the accessory or run position, the parameter should display 'Active'. If it shows 'Inactive', the problem is likely upstream of the BCM (e.g., the ignition switch). If it shows 'Active' but the code is set in the TCM, the problem is likely between the BCM and the TCM/ECM on Circuit 5985.

Wiring & Ground Locations

• Circuit 5985 — A violet wire running from the BCM to the ECM and TCM. It is known to chafe/break in the harness that runs under the driver's side Underhood Fuse Block (X50A) and along the left frame rail.. This is the 'Accessory Wakeup' signal wire. An open or short on this wire is the most common cause of P2537 on this platform.
• BCM Connector X6, Pin 1 — This is the source of Circuit 5985 from the Body Control Module, which is located behind the driver's side of the instrument panel.. This is the starting point of the circuit. A good voltage reading here but a bad reading at the ECM/TCM confirms a wiring issue between the modules.
• ECM Connector X1, Pin 43 — The Engine Control Module is located in the engine compartment. This is the destination pin for Circuit 5985.. This is a key point for testing voltage. No voltage here while the BCM output is good points to a break in the harness.
• TCM Connector X1, Pin 48 — The Transmission Control Module is located on the transmission. This is another destination pin for Circuit 5985.. This is another primary test point. The TCM is the module that most commonly reports this specific DTC.
• X50A — The Underhood Fuse Block, located on the driver's side of the engine bay.. The wiring harness for Circuit 5985 passes directly underneath this fuse block, which is a primary chafe point leading to wire damage. Terminal issues within the fuse block's connectors can also cause related voltage loss issues.
• G110 / G114 — Main body and engine harness grounds, typically located on the frame or engine block.. While not directly on Circuit 5985, a poor ground connection for the BCM or other modules can cause floating voltages and trigger various electrical codes, including P2537.

Real Owner Repair Stories

• gm-trucks.com forum user (2014 GMC Sierra) — Blower motor stayed on after the truck was turned off, Check Engine Light on with code P2537.
  ❌ Tried (didn't work) Initial diagnosis pointed towards blower motor resistor, but the code P2537 suggested a different root cause.
  ✅ What actually fixed it The owner located the large wiring harness under the driver-side underhood fuse block (X50A). After unwrapping the harness, they found a broken purple (violet) wire, which is Circuit 5985. They repaired the wire with a butt connector and heat shrink tubing, which resolved all symptoms and cleared the code.