P1682 on 2002-2009 Chevrolet TrailBlazer: Ignition Circuit Fault Causes and Fixes

What's Unique About the 2002-2009 Chevrolet TrailBlazer

Unlike many vehicles where this code might point to the ignition switch, on the GMT360 platform (which includes the TrailBlazer, Envoy, Rainier, etc.), the problem is notoriously linked to the Underhood Fuse Block (UBEC). A specific, thin 20-gauge solid copper wire inside the multi-layered fuse block is prone to breaking from heat cycles and vibration. This manufacturing and design flaw leads to an intermittent open circuit that causes the exact voltage discrepancy the P1682 code looks for. This issue is so well-documented in owner forums and on YouTube that repairing the fuse block has become a common, platform-specific procedure for this code.

Symptoms You May Notice

• Engine turns over but will not start (crank-no-start).
🎬 Watch: A real-world diagnosis of this crank-no-start condition.
• Engine stalls intermittently, especially after it has warmed up and the broken wire separates.
• Vehicle may start fine when cold but fails to start when hot.
• Check Engine Light is illuminated.
• 'Reduced Engine Power' message may appear on the dash.

Most Likely Causes

1. Internal Failure of the Underhood Fuse Block (UBEC) 🔴 High Probability → Shop Fuse Box A specific 20-gauge solid copper wire inside the fuse block is known to break over time due to thermal stress and vibration, causing an open circuit. This is a widely documented failure point for the TrailBlazer and related GM GMT360 platforms.
   How to confirm: After ruling out relays, diagnosis involves removing the fuse block, disassembling it (which requires drilling out plastic rivets with a 1/2-inch bit), and visually inspecting the internal circuit board for the broken wire. Continuity testing the related pins can also confirm the internal break.
   Typical fix: Since new fuse blocks are often discontinued, the fix is to solder a new piece of 20-gauge wire to bridge the break. Alternatively, a used fuse block can be sourced from a junkyard for around $30.
   Est. part cost: $1-$85 (for wire or a used part)
2. Faulty Powertrain or Ignition Relay 🟡 Medium Probability → Shop Wiring Relay
   How to confirm: The relays are located in the underhood fuse block. The easiest way to test is to swap the suspect relay (often labeled 'PWR/TRN' or 'RUN/CRK') with an identical relay from a non-critical circuit (like the horn or rear defogger). If the problem is resolved, the relay is bad.
   Typical fix: Replace the faulty relay.
   Est. part cost: $10-$30
3. Faulty Ignition Switch ⚪ Low Probability → Shop Ignition Switch While less common on this platform, it's still a possible cause.
   How to confirm: Requires testing the voltage outputs at the ignition switch connector with a multimeter while cycling the key through its positions. The wiring diagram for the vehicle is necessary to identify the correct wires. Many owners replace this part first, only to find the P1682 code persists because the fuse block was the real issue.
   Typical fix: Replace the ignition switch.
   Est. part cost: $40-$100

Rare But Worth Checking

• Broken Wire in Harness: A wire can break anywhere between the fuse block and the ECM. One owner on YouTube documented finding a break in the yellow wire leading to the powertrain relay control circuit by using a multimeter to check for continuity from the ECM connector to the relay socket. This requires meticulous tracing and continuity testing with a wiring diagram.
• Faulty Engine Control Module (ECM): → Shop Engine Control Module (ECM) This is very rare and should only be considered after all other possibilities, especially the fuse block, have been thoroughly exhausted. This fault is often accompanied by other communication codes (U-codes) and may be suspected if a known-good fuse block doesn't resolve the issue. In a cross-manufacturer example, NHTSA ODI #11102191 notes a case where an ECM was replaced for P1682, but the failure recurred.

Diagnosis Steps

1. Check for other stored DTCs. Address any other codes, especially P0689, as it strongly points towards the same root cause.
2. Inspect and test the battery and charging system to ensure proper voltage is available.
3. Locate the underhood fuse block. Swap the Powertrain relay (PWR/TRN) and Run/Crank relay with identical relays from a different circuit (e.g., horn) and see if the vehicle starts. If it does, replace the bad relay.
4. If relays are good, the fuse block is the primary suspect. Disconnect the negative battery terminal.
5. Remove the underhood fuse block. This involves disconnecting several large connectors underneath and unbolting it from the chassis.
6. Disassemble the fuse block. This is a complex process that involves drilling out the heads of the plastic welds/rivets (a 1/2-inch drill bit works well) 🎬 See how to safely take apart the fuse block layers. to separate the layers of the circuit board. Be sure to use a vacuum to clean all metal and plastic shavings to prevent future shorts.
7. Carefully inspect the internal wiring for a broken solid copper wire. The break is often in the same location across many vehicles, as shown in multiple YouTube repair videos.
8. If a break is found, use a soldering iron to repair the connection with a new piece of 20-gauge solid copper wire.
9. Reassemble and reinstall the fuse block, clear the codes, and attempt to start the vehicle.

Parts You'll Likely Need

• Underhood Fuse Block (UBEC) (OEM #15141559 (2006 model example), 15141557 (2006), 15210964 (2005), 25925582 (2008-2009). Part numbers vary by year and options.) — This is the most common point of failure for a P1682 code on this platform due to a well-documented internal broken wire. The part is often discontinued from GM, making DIY repair or sourcing a used part 🎬 Watch: Step-by-step guide to replacing the entire fuse box. the primary solutions.
  Trusted brands: ACDelco (OEM), Used OEM from salvage yards
  OEM price range: N/A (Often Discontinued)
  Aftermarket price range: $30-$150 (Used)
• Powertrain Relay — This relay provides one of the key voltage signals to the ECM and is a common, inexpensive failure point that can trigger P1682.
  Trusted brands: ACDelco, Bosch, Standard Motor Products
  OEM price range: $15-$30
  Aftermarket price range: $10-$20

Related Codes That Often Appear With This One

• P0689 (Engine Controls Relay Feedback Circuit Low Voltage): This code is very commonly seen with P1682 because the same internal fuse block failure can affect both the ignition voltage sense circuit and the engine control relay feedback circuit.
• P2138 (Accelerator Pedal Position Sensor): As noted in NHTSA ODI #11623165, this code can sometimes appear alongside P1682, resulting in a condition where the vehicle runs but will not accelerate.

Platform-Specific Known Issues

• The underhood fuse block on the 2002-2009 TrailBlazer and its GMT360 platform mates is highly prone to an internal broken 20-gauge copper wire that specifically causes code P1682, often accompanied by P0689.
• The original equipment fuse block has been reported as discontinued by GM, making DIY repair or sourcing a used part the primary solutions.
• The failure is often heat-sensitive, causing a no-start condition when the engine bay is hot and allowing a normal start once it has cooled down completely.
• NHTSA ODI #11712762 highlights that even after fuse box inspection and ignition switch replacement, P1682 can return months later, sometimes leading to a "limp mode" condition.

Mechanic-Grade Diagnostic Values

• Voltage difference between Ignition 1 Voltage and Engine Controls Ignition Relay Feedback Signal parameters on a scan tool. — expected: Less than 3.0 Volts.. Failure: A difference greater than 3.0 Volts for one second will set DTC P1682.
• Resistance in either Ignition 1 voltage circuit. — expected: Less than 5.0 Ohms.. Failure: Resistance of 5.0 Ohms or greater can be enough to set the code.
• Continuity check on the yellow wire between the ECM connector (Pin 40) and the powertrain relay socket (Pin D1) in the fuse block. — expected: Near 0 Ohms.. Failure: An Open Link (OL) reading on a multimeter indicates a broken wire in the harness.

Scan Tool Commands That Help

• Tech2 / High-End Scan Tool: Live Data Monitoring: 'Ignition 1 Voltage' and 'Engine Controls Ignition Relay Feedback Signal' — This is the primary diagnostic step. Observing these two parameters with the key on allows a technician to see the voltage discrepancy that the ECM is reporting, confirming the fault condition in real-time.

Wiring & Ground Locations

• Underhood Fuse Block Connector C1 — Gray 68-way connector on the underside of the underhood fuse block.. This connector carries critical power and signal wires between the main harness and the fuse block's internal circuits. A poor connection here can mimic an internal fuse block failure.
• Underhood Fuse Block Connector C2 — Black 68-way connector on the underside of the underhood fuse block.. Pin F9 on this connector is a Pink wire for Ignition 1 Voltage, which is one of the circuits monitored by the ECM for code P1682.
• G103 — Located at the left rear of the engine compartment on the cowl, above the brake booster.. This is a major ground point for the Body Control Module (BCM) and Instrument Panel Cluster (IPC). While not a direct cause, a poor ground here can cause a variety of bizarre electrical issues that could complicate diagnosis.
• G105 — Located at the left front of the engine compartment, behind the cooling fan relays.. This ground serves several components in the front of the engine bay. Verifying major grounds like this are clean and tight is a good practice when diagnosing any electrical fault.
• Fuse #28 (PWR/TRN) — In the Underhood Fuse Block. For L6 engines (2002-2003), this is labeled as 'Powertrain Control Module 1'.. This fuse supplies power to the powertrain relay, which provides one of the two ignition voltage signals to the ECM.
• Fuse #23 (ETC) — In the Underhood Fuse Block. Labeled 'Electronic Throttle Control'. On some GM vehicles, this fuse is labeled 'ECM/Throttle Cnt'.. This fuse is related to the Throttle Actuator Control (TAC) system, which gets its power from one of the ignition circuits monitored for P1682. A loose connection at this fuse has been reported to cause the code.

Real Owner Repair Stories

• YouTube user 'Alan Sunday' (2007 Chevrolet Trailblazer SS (6.0L V8), 120,000 miles) — Engine turns over but will not crank, especially when hot. Later progressed to stalling while driving when hot. Code P1682 was present.
  ❌ Tried (didn't work) Replaced the ignition switch., Replaced the battery., Replaced the Powertrain and Starter relays.
  ✅ What actually fixed it The owner disassembled the underhood fuse block and found the known broken 20-gauge solid copper wire. He soldered a new piece of wire to repair the break, which resolved all symptoms.
• YouTube user 'Shaun's Garage' (2009 Chevrolet Trailblazer (4.2L I6)) — Crank no-start, code P1682.
  ❌ Tried (didn't work) Initial suspicion was the fuel pump, but it could be heard priming.
  ✅ What actually fixed it Instead of the fuse block itself, the technician found a break in the yellow signal wire in the harness between the ECM (pin 40) and the powertrain relay socket in the fuse block (pin D1). He confirmed the open circuit with a multimeter and repaired the wire, fixing the no-start condition.
• NHTSA ODI #11442437 — An owner reported the vehicle left them on the side of the road three times in one day with code P1682. Despite replacing the ignition relay switch three times, the failure persisted.
• NHTSA ODI #11354020 — One owner reported that after their vehicle shut off, it would not start back up; an OBD2 scan revealed P1682 (noted as a faulty ignition switch) and U0101.

OEM Part Supersession History

• 15210964 → 15141559 — Standard part update/revision.
  Heads up: Part number 15210964 is noted as being interchangeable with 15141559 since April 2006 for the U.S. market. Part 15141559 fits 2005-2006 V8 models (5.3L, 6.0L). Always verify the part number against the vehicle's VIN, as different engine and option packages used different fuse blocks.

Model Year Variations Within This Range

• 2002-2009: While the internal broken wire is a common failure across the generation, the specific OEM part number for the underhood fuse block varies by model year and engine (L6 vs V8). For example, 15141559 is commonly cited for 2006 V8 models. A fuse block from a 4.2L I6 model may have different internal circuitry than one from a 6.0L V8 SS model, even if the external case is the same.

Diagnostic Flowchart

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used Underhood Fuse Block (UBEC) is a very common and sensible choice for this repair, primarily because new OEM parts are often discontinued and unavailable from GM. Given the failure is a specific, repairable broken wire, even a used part can be preemptively repaired and reinforced before installation for long-term reliability.

Donor-vehicle mileage cap: roughly under 120000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Visually inspect all underside connectors for signs of corrosion, melted plastic, or pushed-out pins.
• Check the exterior case for cracks or heat damage.
• If possible, source from a vehicle in a dry climate to minimize the risk of moisture-related corrosion.
• Ensure the part number on the used block matches the original exactly, as V8 and I6 models may have internal differences.
• Before installing, compare every fuse and relay from the old block to the donor block to ensure they match your vehicle's specific options.

OEM-only on this vehicle (don't cheap out):

• Underhood Fuse Block (UBEC) - While a used OEM part is the go-to, aftermarket versions of this complex assembly are not common and would be risky. Sticking with a used OEM part is the standard practice.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0689 — ECM/PCM Power Relay Sense Circuit Low. This is a very common companion code to P1682 on the TrailBlazer. The same internal broken wire in the underhood fuse block that causes P1682 often disrupts the power relay sense circuit, causing both codes to appear simultaneously from the single root cause.
• P0443 — Evaporative Emission System Purge Control Valve Circuit. While not directly related by definition, this code was reported alongside P1682 in one case. It can occur if the internal failure within the underhood fuse block also affects the power or ground circuit for the EVAP system components, which are routed through the same assembly.

Frequently Asked Questions