Got a “Reduced Engine Power” warning and a P1682 code staring you down? This post breaks down exactly what’s happening inside your GM’s electrical system, why it triggers that dreaded limp mode, and what you can do about it — from the ignition switch to the fuse block. Read to the end, because the fix isn’t always what you’d expect.
What Is the Chevy P1682 Code?
The Chevy P1682 code means your Engine Control Module (ECM) spotted a voltage mismatch between two separate ignition power circuits. GM calls it the Ignition 1 Switch Circuit 2 fault — sometimes labeled “Driver 5 Line 2” in service documentation.
Here’s the quick version: your ECM watches two independent power feeds simultaneously. One comes straight from the ignition switch. The other runs through the KR75 Powertrain Relay. When the voltage between these two circuits drifts too far apart, the ECM flags P1682 and often drops the engine into Reduced Engine Power mode to protect itself.
This isn’t just a nuisance code. It’s your truck or SUV telling you its core electrical safety net is failing.
Why GM Uses Two Ignition Circuits
Modern GM vehicles don’t have a cable connecting your foot to the throttle. Instead, they use a Throttle Actuator Control (TAC) system — an electric motor opens the throttle plate based on ECM commands. Because a rogue throttle is dangerous, GM engineered two separate power paths to the ECM so it always has a cross-reference to check.
| Circuit | Power Source | Primary Role |
|---|---|---|
| Circuit 1 | Ignition Switch (Direct) | Baseline logic, wake-up signal |
| Circuit 2 | KR75 Powertrain Relay | TAC motor, fuel injectors, coils |
| Battery B+ | Constant Feed | Keep-alive memory (KAM) |
If one circuit drops while the other stays high, the ECM knows something’s wrong — and it shuts things down before the throttle does something unpredictable.
Common Symptoms of P1682
The symptoms go well beyond a check engine light. Because P1682 hits the circuits that power your engine’s core actuators, the effects are hard to miss.
- “Reduced Engine Power” message — The most common symptom. The ECM limits throttle and sometimes cuts cylinders.
- Stalling while driving — A sudden voltage drop on Circuit 2 can kill the engine without warning.
- Crank but no start — The ECM may refuse to fire coils or injectors when it detects the parity fault.
- Intermittent no-start — The vehicle starts fine one morning and won’t crank the next.
- Traction Control Off / Stabilitrak lights — The stability system disables itself because the ECM can’t guarantee safe torque control.
- Dim headlights — Points to a broader power distribution issue.
| Symptom | Cause |
|---|---|
| Reduced Engine Power Message | TAC Failsafe Mode |
| Stalling While Driving | Voltage Drop on Circuit 2 |
| Crank but No Start | ECM blocking spark/fuel due to parity error |
| Traction Control / Stabilitrak Off | ECM can’t send torque data to ABS module |
| Dim Headlights | Systemic power distribution failure |
The intermittent no-start condition is especially frustrating. Many owners replace fuel pumps or crankshaft sensors before realizing the problem is electrical — specifically P1682.
The Three Most Likely Causes
1. A Worn Ignition Switch
The ignition switch isn’t a simple toggle. It’s a multi-contact assembly that routes power to different systems as the key rotates. Over time, those copper contacts pit, oxidize, and develop carbon tracking — a high-resistance path that drops voltage right where the ECM monitors it.
Here’s the tricky part: your radio works, the engine cranks, but the ECM’s specific ignition feed sits at 9V while the battery reads 12.6V. That gap triggers P1682 every time.
2. A Failing KR75 Powertrain Relay
The KR75 relay provides Circuit 2’s high-current feed. Like all relays, its internal contacts arc and pit over time. Cold weather makes this worse — moisture can freeze on the contacts and create a resistance barrier that blocks full voltage from reaching the ECM.
This was a known issue on the Chevrolet Cruze and Impala, which led GM to develop an updated relay with better contact metallurgy. If your relay’s contact resistance spikes past the ECM’s threshold — even briefly — P1682 fires.
3. The Underhood Fuse Block (UBEC)
The underhood bussed electrical center houses the relays and fuses, but it also contains embedded metal busbars that carry power. Two specific failure modes matter here:
- Loose terminal tension — The female terminals that grip relay pins lose spring force from heat and vibration. A loose pin equals a voltage drop.
- Internal corrosion — Moisture sneaks between the UBEC’s internal layers, especially in salt-belt states. The corrosion is invisible from outside but creates a resistive path that drops voltage on the ignition circuit.
On the Chevrolet Traverse, GM identified a fuse block mounting issue where vibration — like hitting a pothole — disrupted the internal busbars and triggered P1682 along with Stabilitrak and traction control warnings.
GM Technical Service Bulletins for P1682
GM has published specific TSBs that fast-track the diagnosis depending on your vehicle. Check these before you start swapping parts.
| Model | TSB | Problem | Fix |
|---|---|---|---|
| Traverse / Acadia / Enclave (2013–2017) | 19-NA-276 | Terminal 51 tension in X3 connector | Replace lead (P/N 84757974) |
| Silverado / Sierra 1500 (2019–2021) | 21-NA-032 | H6 pin dual-crimp in UBEC X3 | Repair terminal connection |
| Silverado / Sierra (2014–2015) | PIT5253B | KR75 Relay #70 internal resistance | Replace relay (P/N 13595908) |
| Cruze / Impala | Mitchell1 / GM | Frost on relay contacts | Replace relay (P/N 13595908) |
One important note for 2019–2021 Silverado and Sierra owners: If P1682 sets alongside a P0101 (Mass Air Flow Performance) code, don’t replace the MAF sensor. TSB 21-NA-032 identifies the root cause as a compromised dual-crimp at pin H6 of the X3 connector. Both codes share a power distribution point at that terminal — one loose wire triggers both faults.
How to Diagnose P1682 Correctly
Static resistance tests (ohmmeter) often miss P1682 entirely. The circuit can show near-zero resistance with no load, then fail the moment it’s carrying 15 amps. You need dynamic testing.
Step 1: Check Live Data
Use a scan tool to monitor two PIDs simultaneously:
- Ignition 1 Signal (switch side)
- EC Ignition Relay Feedback Signal (relay side)
Both values should stay within 0.2V–0.5V of each other. If the switch shows 13.8V and the relay feedback shows 11.2V, the fault is in the relay, the relay’s fuse, or its wiring — not the switch.
Step 2: Dynamic Voltage Drop Test
Probe across the ignition switch — input (red wire) to output (pink wire) — while the key is in the Run position. A voltage drop over 0.3V across those contacts points to internal switch degradation.
For the relay, probe terminal 30 (B+ input) against terminal 87 (switched output). They should match within a few tenths of a volt when the relay is energized.
Step 3: Stress-Test the Fuse Block
If both the switch and relay pass, physically wiggle the X3 connector on the UBEC while watching your scan tool. If the relay feedback voltage dips when you move the harness, you’ve found a terminal tension or crimp issue inside that connector.
For the Lambda-platform vehicles — Traverse, Acadia, Enclave — do a terminal drag test on cavity 51. Insert a test pin and feel for resistance. If the pin slides freely, the terminal’s too loose and it’ll keep causing P1682.
KR75 Relay Terminal Reference
| Terminal | Function | Expected Reading (Ignition On) |
|---|---|---|
| 30 | Battery Feed (B+) | Constant 12.6V+ |
| 87 | Switched Output | 12.6V+ (should match terminal 30) |
| 85 | Coil Positive | 12.6V+ |
| 86 | Coil Ground (ECM-controlled) | 0V |
The relay coil’s resistance (between terminals 85 and 86) should fall between 70–110 ohms or 200–250 ohms depending on the application. Outside that range? The coil’s failing and can’t snap the contacts shut with enough force.
Repair Options That Actually Work
Replace the Relay First (2014–2015 Silverado/Sierra)
If TSB PIT5253B applies to your truck, swap the KR75 relay with updated part number 13595908. It’s cheap, it’s fast, and it resolves the internal resistance issue GM identified on these specific model years.
Repair the Terminal, Not the Harness
For X3 connector issues (Terminal 51 or H6), GM supplies terminated lead kits so you replace one pin — not the entire engine harness. Crimp properly and seal with heat-shrink tubing. Moisture is the enemy.
The Washer Fix for Traverse Fuse Blocks
If your Traverse P1682 appears after bumps or rough roads, the underhood fuse block mounting screws may not be clamping the internal busbars tightly enough. Adding two small washers to the mounting screws increases the clamping force and stops the intermittent contact loss — a well-documented field repair that saves you from buying a new fuse block.
Use Dielectric Grease on Every Connector You Touch
It doesn’t conduct electricity — it seals out moisture and oxygen. Apply it to every terminal you open during this repair. This matters especially if you’re in a salt-belt state where brine works its way into harnesses and causes corrosion years after a repair looks finished.
How the ECM Decides to Set P1682
The ECM doesn’t set P1682 on a single bad reading. It uses a statistical fault confirmation method — typically 20 failures out of 25 consecutive 250-millisecond samples. That’s why the code feels intermittent. The fault may exist, but it clears before the sample window fills.
P1682 is a Type B diagnostic code, which means:
- The MIL (check engine light) won’t illuminate on the first failure.
- It needs two consecutive drive cycles with the fault present before the light comes on.
- But the “Reduced Engine Power” message can appear on the first failure to protect the engine.
That’s why some owners see the limp mode message without a check engine light — the ECM’s protecting the throttle system before the fault is officially “confirmed.”
The monitoring only runs when:
- The powertrain relay is commanded ON
- System voltage stays between 11.0V and 18.0V
- The engine is cranking or running
Outside those conditions, the diagnostic pauses — which is why extreme cold-weather starting events don’t generate false P1682 codes.
