P1682 on 2014-2021 GMC Sierra 1500: Ignition Voltage Fault Causes and Fixes

What's Unique About the 2014-2021 Gmc SIERRA 1500

On this generation of GMC Sierra and its platform mates, the P1682 code is very frequently traced back to poor connections or corrosion within the Underhood Bussed Electrical Center (UBEC). Several Technical Service Bulletins (TSBs) point specifically to compromised terminals within the large connectors that attach to the bottom of the fuse block. TSB 21-NA-032 identifies a faulty wire crimp at connector X3, pin H6 as a primary culprit that can also trigger a P0101 MAF code. This makes inspecting the fuse block connectors a critical first step, as it is a well-documented pattern failure 🎬 Watch: Step-by-step guide to replacing a faulty GM fuse box. on these trucks.

Diagnostic Flowchart

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

Generation note: This year range covers the third generation (2014-2018 K2XX) and the fourth generation (2019-2021 T1XX) of the GMC Sierra 1500. The underlying cause, often related to terminal issues in the underhood fuse block, is common to both generations, as confirmed by TSBs covering both platforms. However, specific connector and terminal locations (e.g., X2, X3, X4) may differ, so consulting the correct service manual for your model year is important. TSB 21-NA-032 specifically calls out connector X3, pin H6 for inspection on 2019-2021 models.

Symptoms You May Notice

• Crank but no start condition.
• Intermittent no-crank or no-start.
• Engine shuts off while driving.
• Engine continues to run even after turning the ignition off.
• Check Engine Light is on.
• Reduced Power Mode may be activated.
• Unable to program the Engine Control Module (ECM).
• Service StabiliTrak / Traction Control warning message may appear.

Most Likely Causes

1. Poor Connection at Underhood Fuse Block (UBEC) 🔴 High Probability TSBs for this platform specifically identify compromised, corroded, or backed-out terminals in the UBEC connectors as a primary cause. TSB 21-NA-032 points to a faulty dual-crimp wire at connector X3, pin H6. Vibration can also loosen the main bolts securing the connectors to the bottom of the fuse block, causing intermittent contact.
   How to confirm: Disconnect the battery. Unbolt and lift the underhood fuse block to inspect the large connectors underneath. Pay extremely close attention to connector X3, pin H6 for any signs of corrosion, backed-out pins, or a loose wire crimp. Also check that the bolts securing the connectors are tight. A simple test is to wiggle specific fuses (like the 'ECM throttle count' fuse) with the engine running; if the engine stumbles or stalls, the connection in the fuse block is poor.
   Typical fix: Clean corroded terminals using a contact cleaner and a small brush. If a terminal is damaged, it must be replaced using a specific terminal repair kit (e.g., GM P/N 19332366 for the X3-H6 terminal). In some cases, simply tightening the loose bolts on the underside connectors resolves the issue. If corrosion is widespread or an internal wire is broken, the entire fuse block may need replacement.
   Est. part cost: $10-$30 for a terminal repair kit, $150-$350 for a new fuse block.
2. Faulty Ignition or Powertrain Relay 🟡 Medium Probability
   How to confirm: Swap the Powertrain (KR75, #70) and Run/Crank relays with known good relays of the same type (e.g., the horn relay) to see if the problem resolves. A GM service document specifically identifies the KR75 relay as a potential cause for P1682 on 2014-2015 models.
   Typical fix: Replace the faulty relay. The specific Engine Controls Ignition Relay #70 may be part number 13595908 or 13500118.
   Est. part cost: $10-$30
3. Damaged Wiring ⚪ Low Probability GM has identified several areas where the engine wiring harness can chafe against chassis and engine components, causing shorts. Common chafe points include the corner of the ECM/TCM bracket, the driver side upper control arm, and the shock tower bolt.
   How to confirm: Perform a continuity and voltage drop test on the ignition voltage circuits between the fuse block and the ECM. A resistance of 5 ohms or more can trigger the code. Visually inspect the wiring harness for chafing in the areas identified in service bulletins, particularly behind the driver's side front wheel well liner.
   Typical fix: Repair the damaged section of the wire harness using splice connectors and heat shrink. Reroute and secure the harness with anti-abrasion tape and zip ties to prevent future damage.
   Est. part cost: $5-$25 for wiring repair supplies.
4. Faulty Ignition Switch ⚪ Low Probability
   How to confirm: Use a multimeter to test the voltage output from the ignition switch circuits in different key positions (Off, Accessory, Run, Crank). The voltage should be stable and consistent according to the vehicle's wiring diagram. A drop in voltage or intermittent signal indicates a failing switch.
   Typical fix: Replace the ignition switch.
   Est. part cost: $40-$100

Rare But Worth Checking

• Faulty Engine Control Module (ECM): This is rarely the cause. The ECM is often replaced unnecessarily when the actual problem is a wiring or connection issue supplying power to it. Before replacing the ECM, all other possibilities, especially the fuse block connections, must be exhaustively ruled out.
• Water Intrusion into Headlamp Assembly: According to TSB PIT5546G, water leaking into the headlamp assembly or its electrical connectors can cause a short circuit. This can keep various modules powered on, drain the battery, and cause voltage irregularities that trigger P1682, especially in wet weather.

Diagnosis Steps

1. Verify the battery is fully charged and passes a load test.
2. Scan for all DTCs. If codes like P0685, P0689, or P0690 are present, diagnose them first as they are directly related to the ECM power relay circuits.
3. Check all fuses related to the ignition and ECM. With the key on, gently wiggle fuses like the 'ECM throttle count' fuse to see if it causes the engine to stumble or the code to appear, indicating a poor socket connection.
4. Swap the 'Powertrain' (KR75/#70) and 'Run/Crank' relays with identical, known-good relays to rule out a simple relay failure.
5. Disconnect the battery. Remove the mounting bolts for the underhood fuse block (UBEC/X50A) and carefully lift it to access the connectors underneath.
6. Check that the connector bolts are tight. A loose bolt can cause an intermittent connection.
7. Thoroughly inspect the terminals in the connectors. Pay special attention to connector X3, pin H6 (a dual-crimp wire) for corrosion, backed-out pins, or damage, as specified in TSB 21-NA-032 for 2019+ models.
8. If corrosion or damage is found, clean or repair the specific terminals. If no visible issue is found, proceed to voltage testing.
9. If wiring chafe is suspected, inspect the harness near the ECM/TCM bracket, upper control arm, and shock tower.
10. If fuse block and wiring are confirmed good, test the ignition switch outputs to ensure it is sending the correct signals.

Parts You'll Likely Need

• Underhood Fuse Block Terminal (OEM #19332366 (For X3-H6)) — This is the most common failure point according to GM TSBs. A single corroded or loose terminal, especially at X3-H6, can cause the P1682 code.
  Trusted brands: ACDelco
  OEM price range: $10-$30
  Aftermarket price range: $5-$20
• Underhood Fuse Block (UBEC) (OEM #84303175) — If corrosion is widespread or the internal circuit board of the fuse block is damaged, the entire assembly must be replaced. Part 84303175 is an updated design that supersedes several previous part numbers.
  Trusted brands: ACDelco, Dorman
  OEM price range: $250-$400
  Aftermarket price range: $150-$250
• Engine Controls Ignition Relay (KR75/#70) (OEM #13595908) — A faulty relay can cause an intermittent crank/no-start condition with P1682, as identified in GM service documents for 2014-2015 models.
  Trusted brands: ACDelco
  OEM price range: $15-$30
  Aftermarket price range: $10-$20

Related Codes That Often Appear With This One

• P0101 — According to TSB #21NA032, a compromised connection in the underhood fuse block at terminal X3-H6 can cause both P1682 and P0101 to set simultaneously.
• P0685 — This code relates to the ECM/Powertrain Control Module Power Relay Control Circuit. Since P1682 involves a discrepancy in power supplied by relays, a fault in the relay control circuit can be a related issue.
• P0689 — This code indicates a low voltage condition in the ECM/PCM Power Relay Sense Circuit. It often appears with P1682 because both point to a problem in the ECM's power supply and is mentioned in TSBs alongside P1682.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-032: Warns that P1682 and P0101 can be set by a compromised dual-crimp wire at the UBEC X3 connector, position H6. Explicitly advises against replacing the MAF sensor.
• PIT5546G: Links P1682 with a no-start or dead battery condition caused by water intrusion into a headlamp assembly or its connector, leading to an electrical short.
• GM Document (similar to PIT5253B): For 2014-2015 models, suggests that an intermittent crank/no-start with P1682 can be caused by a faulty KR75 Engine Controls Ignition Relay #70 and provides a specific part number for replacement.
• 21-NA-149: Details common wiring harness chafe points on 2019-2021 trucks that can lead to various electrical issues, including no-start conditions.

Platform-Specific Known Issues

• Wiring Harness Chafing: On 2019-2021 models in particular, GM issued bulletins (like 21-NA-149) regarding the engine wiring harness rubbing through on sharp edges of the ECM/TCM bracket, the upper control arm, or the shock tower. This can cause a multitude of electrical problems, including shorts that could lead to a P1682 code and a no-start condition. Inspection requires removing the driver's side wheel and wheelhouse liner.

Mechanic-Grade Diagnostic Values

• Voltage difference between 'Ignition 1 Voltage' and 'EC Ignition Relay Feedback' on a scan tool — expected: Less than 3.0 Volts. Failure: A difference greater than 3.0 Volts for more than one second will set the code. A real-world failure example showed one circuit at 11.8V and the failing circuit at 1.4V.
• Resistance of Ignition 1 voltage circuits — expected: Less than 5.0 Ohms. Failure: A resistance of 5.0 Ohms or greater can cause high resistance and trigger the DTC.
• Voltage at ECM Connector C1, Pin 19 (Ignition 1 Voltage Circuit) — expected: Battery Positive Voltage (B+). Failure: Voltage significantly less than B+ indicates an open or high resistance in the circuit between the fuse block and the ECM.

Hidden / Shadow Codes Worth Checking

• P16A7, P16BC: These are additional DTCs related to ECM internal performance that may be stored in history along with P1682 and P0101, pointing towards the UBEC connection issue. (see via A professional scan tool like GM GDS2, checking historical codes.)

Scan Tool Commands That Help

• GM GDS2 / Professional Scan Tool: Live Data Monitoring: 'Ignition 1 Voltage' and 'EC Ignition Relay Feedback' — This is the primary diagnostic method. These two parameters must be monitored with the key on, engine off. They represent the two ignition feeds to the ECM, and their values should be nearly identical. A significant discrepancy confirms the fault condition. Note: 'EC Ignition Relay' is the scan tool's term for the powertrain relay.

Wiring & Ground Locations

• X50A — The Underhood Bussed Electrical Center (UBEC), commonly known as the underhood fuse block, located on the driver's side of the engine compartment.. This is the central hub for the fault. The problem is almost always a poor connection at one of the large connectors on the underside (like X3) or an internal failure of the block itself.
• Connector X3, Pin H6 — A specific pin location within a large connector on the underside of the X50A fuse block.. TSB 21-NA-032 specifically identifies a compromised dual-crimp wire at this pin as the root cause for P1682, especially when P0101 is also present on 2019-2021 models.
• G103 / G107 — A primary body ground point located on the cowl in the left rear of the engine compartment, often near the brake booster.. While not the ECM's primary ground, a poor body ground at this location can cause a host of unpredictable electrical issues and voltage discrepancies across various modules. Verifying major grounds is a key step in any electrical diagnosis.
• ECM Main Ground — Typically a braided strap or wire from the wiring harness to the engine block or cylinder head, often near the thermostat housing on GM V8s.. A compromised ECM ground can cause incorrect voltage readings and erratic behavior. The ECM relies on a solid ground as a reference point for all its voltage measurements.

Real Owner Repair Stories

• Comment on YouTube video 'Causes and Fixes GM / Chevy P1682 Code' (GM Truck (unspecified year)) — Intermittent P1682 code.
  ❌ Tried (didn't work) Initial diagnosis was inconclusive.
  ✅ What actually fixed it The owner removed the underhood fuse box, inspected all pins and connections, and found the bolts holding the wire connectors to the bottom of the fuse box were loose. The final fix was reinstalling the bolts with Loctite to prevent them from loosening again.
• YouTube video 'P1682 code Trailblazer SS turns over, but doesn’t crank pt 1' (2007 Trailblazer SS (uses a similar fuse block design)) — Turns over but won't start, especially when hot. Code P1682.
  ❌ Tried (didn't work) Replaced ignition switch, Replaced battery, Replaced Powertrain and Starter relays
  ✅ What actually fixed it The owner disassembled the multi-layered fuse block and found a broken 20-gauge copper wire inside. The heat from the engine caused the broken wire ends to separate, creating an open circuit. Repairing this internal wire solved the problem permanently.
• YouTube video 'Express Van Intermittent Crank No Start Diagnosis P0689 P1682' (2017 Chevy Express Van) — Intermittent crank no-start with codes P1682 and P0689. Scan tool showed Ignition 1 Voltage at 11.8V but EC Ignition Relay Feedback at only 1.4V.
  ❌ Tried (didn't work) Swapping the powertrain relay, Inspecting the relay for burnt contacts
  ✅ What actually fixed it Voltage tests at the fuses confirmed battery voltage was entering the fuse block but low voltage was coming out on the affected circuit. This isolated the fault to an internal problem within the fuse block's bus bars or layers. Replacing the entire underhood fuse block assembly resolved the issue.

"I Checked Everything" — The Actual Cause

• P1682 is an electrical fault, so a smoke test is not applicable. The equivalent scenario for this code is when external wiring tests (continuity, voltage drop) from the fuse block to the ECM appear normal, yet the problem persists. In these cases, the root cause is often an intermittent failure *inside* the fuse block itself, such as a loose connector bolt, a poor terminal connection that only fails when hot, or a cracked internal trace/wire that cannot be seen without disassembling the block.

OEM Part Supersession History

• 23285572, 84008524, 84071500, and others → 84303175 — Design change. The new part may include extra content and replaces older fuse blocks that had fewer features or a different internal design.
  Heads up: While the new part is the correct replacement, it's critical to order by VIN to ensure the correct configuration, as some vendors require it. The new block may have fuses or relays in different locations than the original, so the old block should be used as a reference to transfer fuses to their correct spots.

Model Year Variations Within This Range

• 2019-2021 (T1XX Platform): TSB 21-NA-032, which specifically identifies the X3-H6 connector pin as a failure point when P1682 and P0101 are present together, is written for the newer T1XX platform trucks.
• 2014-2018 (K2XX Platform): While the root cause is often the same (fuse block connection), the specific connector and pin numbers may differ from the T1XX platform. The superseded fuse block part number 84303175 is listed as a replacement for parts from this generation, indicating a common failure but potentially different original part numbers.