P1682 on 2003-2007 Cadillac CTS: Ignition Circuit Fault Causes and Fixes

What's Unique About the 2003-2007 Cadillac CTS

For General Motors vehicles of this era, including the Cadillac CTS, the P1682 code is frequently traced back to the underhood fuse box, technically called the Underhood Bussed Electrical Center (UBEC). While the ignition switch can be a cause, many owners and technicians find that the issue is actually a poor terminal connection for a fuse or relay, or internal corrosion/breakage within the fuse box itself. GM has issued service bulletins for other models (like TSB 21-NA-032) that identify a 'compromised connection within the UBEC' as the cause for P1682, confirming this is a known design weakness. This can lead to misdiagnosis, where the more expensive ignition switch is replaced without fixing the underlying problem.

Symptoms You May Notice

• Engine will not start or has a 'crank, no-start' condition.
• Engine stalls intermittently, sometimes while driving, especially when hot.
• Check Engine Light is illuminated.
• Reduced Engine Power message may display.
• The battery may die overnight if a relay remains stuck on.
• Other warning lights (ABS, Traction Control, Stabilitrac) may appear erroneously when the fault occurs.
• Engine continues to run even after the key is turned off and removed (in some cases on related platforms).

Most Likely Causes

1. Poor Connection or Faulty Underhood Fuse Box (UBEC) 🔴 High Probability → Shop Fuse Box The design of the UBEC on many GM vehicles can lead to loose terminal connections or internal corrosion and broken circuits over time, often exacerbated by heat. This is a widely documented failure point.
   How to confirm: With the key on, gently wiggle the fuses and relays labeled 'ECM', 'IGN', or 'PWR TRN' in the underhood fuse box. If this causes the engine to stall or start, the fuse box connection is the issue. A more precise method is to use a scan tool to monitor the 'Ignition 1 Voltage' and 'Engine Controls Ignition Relay Feedback' parameters. A sudden drop in one of these values when wiggling components confirms an internal UBEC fault.
   Typical fix: First, try swapping the relevant relays. If the problem persists, the fuse box must be repaired or, more commonly, replaced. Repair involves disassembling the box to fix the broken internal connection, while replacement is a direct swap.
   Est. part cost: $5-$25 for a relay; $150-$400 for a new fuse box.
2. Faulty Powertrain or Run/Crank Relay 🟡 Medium Probability → Shop Wiring Relay Relays are mechanical switches that wear out over time and are a common failure point in any vehicle's electrical system.
   How to confirm: Locate the powertrain ('PWR/TRN') or ignition ('IGN-1') relay in the underhood fuse box. Swap it with an identical relay from a non-essential system (like the horn or rear defogger). If the car starts and runs reliably, the relay was bad.
   Typical fix: Replace the faulty relay with a new one.
   Est. part cost: $5-$25
3. Faulty Ignition Switch 🟡 Medium Probability → Shop Ignition Switch The ignition switch is a high-use mechanical and electrical part. Its internal contacts can wear down, leading to erratic voltage signals.
   How to confirm: This requires testing the voltage outputs on the various wires of the ignition switch connector with a multimeter while turning the key. A wiring diagram is necessary to identify the correct wires and expected voltages. This should be done after ruling out the more common fuse box and relay issues.
   Typical fix: Replace the ignition switch. This involves removing lower dash panels and the steering column shroud.
   Est. part cost: $40-$150
4. Wiring Harness Issue (Open/Short) ⚪ Low Probability Wires can chafe against engine or chassis components over many years, or corrosion can develop in connectors, leading to a break in the circuit.
   How to confirm: Visually inspect the wiring harness running from the ignition switch, to the fuse box, and to the ECM. Perform a continuity test on the specific ignition circuit wires using a multimeter if a visual fault isn't found.
   Typical fix: Repair the broken or shorted wire and protect the harness from future damage.
   Est. part cost: $5-$20 for repair materials.

Rare But Worth Checking

• Faulty Engine Control Module (ECM): → Shop Engine Control Module (ECM) This is rare and should only be considered after all other possibilities (wiring, fuses, relays, ignition switch) have been exhaustively tested and ruled out. An ECM failure for this code is uncommon.
• Water Intrusion in Headlamp Assembly or Connectors: On some GM platforms, water getting into the headlamp assemblies or their electrical connectors can short circuits and back-feed voltage into unrelated systems, including switched ignition circuits. This can cause modules to stay awake, drain the battery, and trigger P1682. GM issued Preliminary Information bulletin PIT5546B for this issue on other models, which could be relevant if the failure is intermittent and occurs in wet weather.

Diagnosis Steps

1. Connect a scan tool. Check for other DTCs. If P0685, P0689, or P0690 are present, diagnose them first.
2. On the scan tool, monitor the 'Ignition 1 Voltage' and 'Engine Controls Ignition Relay Feedback' parameters. Note which one is reading lower or dropping out.
3. Open the underhood fuse box, located on the passenger side of the engine bay. 🎬 Watch: How to locate and check your fuses and relays Visually inspect fuses and relays related to the ignition ('IGN-1') and ECM ('PWR/TRN'). Ensure they are fully seated.
4. With the engine running (if possible) or ignition on, gently wiggle the relays and fuses identified in the previous step. Watch the scan tool data for a change. If wiggling a component causes the voltage to drop or the engine to stall, you have found the source of the poor connection, which is likely the fuse box itself.
5. Identify the relay corresponding to the voltage that dropped out on the scan tool (e.g., Powertrain/Run/Crank relay). Swap it with an identical relay from a non-critical circuit (e.g., horn). If the problem is resolved, replace the faulty relay.
6. If relays are good, the issue is highly likely the fuse box (UBEC). Inspect the main connectors underneath the fuse box for any signs of corrosion or backed-out pins. Replacement of the fuse box is the most common reliable fix.
7. If the fuse box and relays are confirmed to be good, proceed to test the ignition switch. This involves checking for correct voltage output on multiple pins at the switch connector using a multimeter and wiring diagram.
8. If all other components and wiring have been ruled out, the ECM may be faulty, but this is the least likely cause.

Parts You'll Likely Need

• Underhood Fuse Box (OEM #22782849, 15930594) — Internal connection issues within the fuse box (UBEC) are the most common cause of this code on GM vehicles, often failing due to heat and vibration.
  Trusted brands: ACDelco (GM Genuine Parts)
  OEM price range: $250-$400
  Aftermarket price range: $150-$300
• Ignition Relay / Powertrain Relay — A failing relay is a simple and common electrical failure point that can cause the voltage discrepancy seen by the ECM.
  Trusted brands: ACDelco, Bosch, Standard Motor Products
  OEM price range: $15-$30
  Aftermarket price range: $5-$20
• Ignition Switch — The mechanical and electrical contacts inside the switch can wear out, causing intermittent power signals. It's a common misdiagnosis but can be the actual cause.
  Trusted brands: ACDelco, Standard Motor Products
  OEM price range: $80-$150
  Aftermarket price range: $40-$90

Related Codes That Often Appear With This One

• P0685 — This code relates to the ECM/PCM Power Relay Control Circuit. A fault here directly impacts the power supplied to the ECM, which can trigger P1682.
• P0689 — This code indicates a low voltage condition in the ECM/PCM Power Relay Sense Circuit. It often appears with P1682 because they both monitor the ECM's power supply.
• P0690 — This code indicates a high voltage condition in the ECM/PCM Power Relay Sense Circuit, pointing to a similar set of potential electrical issues.

Technical Service Bulletins (TSBs) & Recalls

• GM TSB 21-NA-032: While this bulletin is for newer 2019-2021 GM trucks, it is highly relevant as it addresses DTC P1682 caused by a 'compromised connection within the UBEC (Underhood Bussed Electrical Center).' This demonstrates that GM has officially acknowledged this specific failure mode in their fuse boxes, which is the same root cause commonly found in the 2003-2007 CTS.
• Bulletin #PIP5729: A manufacturer service bulletin notes that customers may comment that the SES lamp is illuminated or was illuminated and turned off, with DTC P1682 (Ignition 1 Switch Circuit 2) active or stored in history.

Platform-Specific Known Issues

• The underhood fuse box (UBEC) on the 2003-2007 CTS is located on the passenger side of the engine compartment, behind the headlight assembly. It is a known weak point where internal connections can break or corrode, often causing intermittent no-start or stalling conditions when the engine bay gets hot.

Mechanic-Grade Diagnostic Values

• Voltage difference between 'Ignition 1 Voltage' and 'EC Ignition Relay Feedback' scan tool PIDs — expected: Less than 3 volts. Failure: A difference greater than 3 volts for 1 second or more will set the DTC.
• Resistance in the ignition circuit within the Underhood Fuse Block (UBEC) — expected: Less than 6 ohms. Failure: A resistance of 6 ohms or greater in the circuit will cause the DTC to set.
• Voltage at Ignition Switch Harness — expected: Battery Voltage (12V+). Failure: Low or no voltage on the Pink (+) wire at the ignition switch harness indicates a problem upstream or with the switch itself.
• Voltage at ECM C1 Connector, Pin 16 (Pink wire) — expected: Battery Voltage with Key On. Failure: Low or no voltage indicates a break in the 'Ignition 1 Voltage' circuit between the UBEC and the ECM.

Scan Tool Commands That Help

• Tech2 / GDS2: Monitor 'Ignition 1 Voltage' and 'Engine Controls Ignition Relay Feedback' PIDs — This is the primary diagnostic step to determine which of the two ignition circuits is dropping voltage, which isolates the fault to either the Run/Crank relay circuit or the Powertrain relay circuit.

Wiring & Ground Locations

• G101 — In the engine compartment, near the left front strut tower, at the bottom of the inner fender well.. This is a primary ground for the Engine Control Module (ECM). A poor connection here can cause erratic ECM behavior and various power-related codes, including P1682.
• G104 — On the right front corner of the vehicle, on the right body rail near the right headlamp.. This ground serves the underhood fuse block. Corrosion or a loose connection here can cause widespread electrical issues originating from the fuse block, mimicking an internal UBEC failure.
• G100 — On the engine block, near the starter motor.. This is another critical engine and ECM ground point. A bad ground here can affect the ECM's ability to get a stable reference, leading to incorrect voltage readings.
• ECM C1, Pin 16 — On the first large connector (C1) of the Engine Control Module, located on the passenger side of the engine.. This pin, with a Pink wire, is the 'Ignition 1 Voltage' input to the ECM. Probing this pin for battery voltage is a key step to verify the integrity of the circuit from the UBEC to the ECM.
• Ignition Switch Harness — At the steering column, connected to the ignition switch.. The Pink wire in this harness should have 12V+ when the key is in the ON position. Testing here helps differentiate between a faulty ignition switch and a downstream wiring or UBEC problem.

Real Owner Repair Stories

• YouTube channel 'Car Faults' (GMC vehicle (platform shares GM electrical architecture)) — Check Engine Light with code P1682, intermittent stalling, or no-start.
  ❌ Tried (didn't work) Attempting to clear the code without finding the physical fault.
  ✅ What actually fixed it The problem was a loose connection for the 'ECM/Throttle Cont' 15-amp fuse in the underhood fuse box. Wiggling the fuse caused the engine to stall and the code to reappear, confirming the fuse box's internal terminal was the issue, not the fuse itself.
• LS1Tech Forum (2006 Cadillac CTS-V) — Horrible vibration and misfire on cylinders 5, 6, 7, and 8 immediately after a car wash while the engine was running.
  ❌ Tried (didn't work) Checking for fuel delivery (smell of fuel was strong).
  ✅ What actually fixed it The suggested fix, though the final outcome was not posted by the user, was to check the two large connectors at the right of the underhood fuse box for water intrusion, dry them out, and check the wiring harness that routes under the radiator to the PCM for moisture. This points to water causing a short in the UBEC or related wiring.

OEM Part Supersession History

• 15930594 → 15930594 (current) — This part number is listed for multiple GM vehicles over a wide range of years, including the 2010-2016 Cadillac SRX, suggesting it is a long-standing design. While it may have had internal revisions, the part number itself has not been superseded.
• 22782849 → Unknown — This is another part number associated with the UBEC for this platform. Cross-referencing is required to determine which VINs use which part.
  Heads up: Always verify the correct part number with the vehicle's VIN before ordering, as different engine options or model years may use different fuse box layouts.

Model Year Variations Within This Range

• 2003: The 2003 model year has slightly different ground point locations and designations compared to the 2004-2007 models. For example, G101 is near the strut tower on a 2003 model, whereas on later models it may be elsewhere. Always use a wiring diagram specific to the exact year.
• 2004-2007: These models share a more consistent wiring and ground location scheme, including the location of the Dash Integration Module and various fuse blocks in the rear of the vehicle that are not present on all trims/years.

Diagnostic Flowchart

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used Underhood Fuse Box (UBEC) is a high-risk purchase and generally not recommended. The primary failure mode for P1682 is an internal break or corrosion within the UBEC itself, a problem often caused by age, heat, and vibration. A used part from a similar-aged vehicle is likely to have or soon develop the same fault. A used part should only be considered as a temporary, low-cost option to confirm the diagnosis before buying a new unit.

Donor-vehicle mileage cap: roughly under 60000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Inspect the underside of the fuse box for any signs of melting, arcing, or corrosion around the large connector terminals.
• Check for a clean, dry donor vehicle. Avoid parts from cars with signs of flood damage or engine bay fires.
• Physically inspect the fuse and relay sockets on top for any signs of blackening, melting, or loose-fitting terminals.
• If possible, get the UBEC from a vehicle that was running and driving and not junked for electrical problems.

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

• Underhood Fuse Box (UBEC)

Aftermarket brands forum-validated for this vehicle:

• ACDelco (GM Genuine Parts) is the most reliable choice for the UBEC.

Brands owners have reported issues with on this vehicle:

• Unbranded or generic aftermarket fuse boxes are not recommended due to the complexity of the part and the high potential for immediate or premature failure.

Real Owner Stories

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

Related OBD-II Codes

• P0685 — ECM/PCM Power Relay Control Circuit. This code often appears with P1682 because it directly relates to the main power relay circuit, which is one of the two circuits monitored by the ECM that can trigger P1682.
• P0689 — ECM/PCM Power Relay Sense Circuit Low. This is a very common companion code, as it indicates a problem with the feedback circuit from the powertrain relay. A bad connection in the fuse box is a frequent cause for both P0689 and P1682.
• P0690 — ECM/PCM Power Relay Sense Circuit High. This code points to the opposite condition of P0689 but shares the same root cause—a fault in the powertrain relay circuit, often due to a stuck relay or a short, which can also trigger P1682.
• P0443 — EVAP Purge Control Valve Circuit. This code can appear with P1682 if the root cause is a faulty underhood fuse box (UBEC), as the failing internal connections can disrupt power to multiple, seemingly unrelated engine control circuits simultaneously.

Frequently Asked Questions