Portal Packet — 2008 Chevrolet Equinox

We performed a full system scan and initial evaluation of your Equinox. The majority of your vehicle’s issues trace back to a single primary problem: the engine coolant temperature (ECT) sensor circuit is malfunctioning. This incorrect temperature signal is confusing both the engine and transmission computers.

Main Findings

• Cooling / ECT Issue (Primary): The ECT sensor circuit is faulty. This affects fuel mixture, misfire detection, and how the transmission decides line pressure and shift timing.
• Thermostat Not Regulating Temperature: The thermostat is likely stuck open, causing the engine to run too cool and reinforcing the ECT problem.
• Transmission Buzz/Whirr in Gear: A buzz/whirr is heard only when in gear, quiet in Park/Neutral. This is consistent with pressure‑control behavior being thrown off by bad torque/temperature data.
• EVAP System Leak: The emissions system has a leak. This turns on the check‑engine light but does not affect how the vehicle drives.
• Passenger Airbag Seat Sensor Fault: The passenger presence system is reporting a plausibility failure, which can affect airbag deployment logic.
• Rear Defrost Electrical Fault: The rear defrost circuit shows a short to battery/open condition.

Recommended Repair Order

1. Diagnose and repair the ECT sensor circuit.
2. Replace the thermostat and verify proper warm‑up behavior.
3. Re‑evaluate transmission noise and behavior after temperature issues are corrected.
4. Perform EVAP smoke testing to locate and repair leaks.
5. Diagnose and repair the passenger seat airbag sensor / harness.
6. Diagnose and repair the rear defrost circuit fault.

Scan excerpt: “Code number: P0115‑00 … Describe: Engine Coolant Temperature (ECT) sensor circuit … Code number: P0128‑00 … Describe: Engine Coolant Temperature (ECT) below thermostat regulating temperature.”

Transmission Noise Pattern

• Buzz/whirr present only when in Drive/Reverse.
• Noise disappears in Park and Neutral.
• Noise is load‑dependent, not simple RPM‑dependent.

This pattern is consistent with pressure‑control solenoid (PCS) buzz, low or unstable line pressure, or torque converter clutch behavior being mis‑commanded due to incorrect torque/temperature data from the engine side.

Misfire Felt, Not Counted

Misfire is felt by the driver, but misfire counters and graphs remain calm after codes were cleared. This is expected behavior when the ECM is in a fallback or open‑loop strategy after a clear — misfire detection can be limited or temporarily disabled, so the misfire is real but not logged.

Post‑Clear Immediate Re‑Flags (True Active Faults)

Other codes (EVAP small/large leak, TCM not programmed, incorrect gear ratio, etc.) are treated as history or cascade faults until primary cooling/ECT issues are resolved.

Primary Failure — ECT Sensor Circuit (P0115)

• Corrupts engine temperature information.
• Leads to incorrect fuel mixture and lean/rough operation.
• Disrupts torque modeling used by the TCM.
• Can disable or limit misfire detection logic.
• Contributes to gear ratio plausibility and “TCM not programmed” style flags.
• Drives the transmission buzz/whirr under load via incorrect pressure commands.

Secondary Failure — Thermostat Stuck Open (P0128)

• Engine runs cooler than intended.
• Reinforces the ECT lie and keeps the system in a “not warmed up” mindset.
• Extends time in enrichment or fallback strategies.

Tertiary Issues — EVAP Leaks

• Small and large EVAP leak codes are present in history.
• These affect emissions and check‑engine light status, not drivability.

Quaternary Issues — PPS / SDM & Rear Defrost

• Passenger presence system plausibility failure affects airbag logic and is safety‑relevant.
• Rear defrost circuit short/open is an electrical comfort/convenience fault.

5.1 ECT Circuit Diagnostic Flow

1. Verify 5V reference at ECT connector.
2. Verify ground integrity at ECT connector.
3. Backprobe ECT signal wire and observe voltage sweep during warm‑up.
4. Compare scan‑reported temperature to IR gun reading at thermostat housing.
5. Inspect ECT connector for coolant intrusion, corrosion, or pin tension issues.
6. Perform harness wiggle test while monitoring signal stability.
7. If wiring and connector pass, replace ECT sensor.
8. If fault persists, perform ECM pinout verification for ECT signal and ground.

5.2 Transmission Buzz/Whirr Decision Tree

1. Confirm noise occurs only in Drive/Reverse and disappears in Park/Neutral.
2. If yes, suspect load‑dependent hydraulic/PCS behavior rather than mechanical idle noise.
3. Check ATF level hot, in gear, on level ground; inspect for aeration/foaming.
4. Verify ECT fault is present — if yes, correct ECT/thermostat issues first.
5. After ECT repair, re‑evaluate noise; if resolved, document as ECT‑driven PCS behavior.
6. If noise persists, monitor PCS amperage and line pressure (if equipment available).
7. Use stethoscope at valve body area to localize PCS or valve train buzz.
8. Escalate to internal transmission inspection only if hydraulic/electronic causes are ruled out.

5.3 EVAP Leak Flow

1. Connect smoke machine to EVAP system.
2. Cap test fuel cap; replace if leak is observed at cap.
3. Command purge and vent solenoids (if supported) and observe smoke routing.
4. Inspect canister, lines, and filler neck for visible leaks.
5. Repair leaks, clear codes, and verify readiness after drive cycle.

5.4 PPS Fault Flow

1. Inspect passenger seat harness and connectors for damage or loose connections.
2. Check seat mat wiring for pinched, broken, or corroded sections.
3. Inspect SDM connector for corrosion or water intrusion.
4. Clear codes and perform key‑cycle test with a passenger in the seat.
5. If fault returns, follow OEM procedure for mat/harness replacement and calibration.

Cooling / ECT

• Backprobe ECT signal and compare to IR gun temperature.
• Verify 5V reference and ground at ECT connector.
• Inspect connector for coolant intrusion and corrosion.
• Inspect harness routing for rub‑through or strain points.
• Replace ECT sensor if wiring tests good.

Thermostat

• Replace thermostat and gasket.
• Refill and bleed cooling system with correct coolant.
• Verify warm‑up curve and operating temperature on scan tool.

Transmission

• Check ATF level hot, in gear, on level surface.
• Inspect fluid for aeration, discoloration, or burnt odor.
• Monitor PCS amperage and commanded line pressure (if supported).
• Use stethoscope at transmission/valve body area during in‑gear operation.
• Re‑evaluate noise after ECT/thermostat repairs.

EVAP

• Perform EVAP smoke test.
• Test fuel cap sealing.
• Check purge and vent solenoid operation.
• Inspect canister and lines for leaks.

PPS / Rear Defrost

• Inspect passenger seat harness and mat wiring.
• Inspect SDM connector for corrosion.
• Inspect rear defrost grid and connectors.
• Test rear defrost relay and power/ground circuits.

Initial diagnostic labor total: 3.9 hr (parts and any follow‑on repair labor to be finalized after testing). “You saved X” style comparison is intentionally omitted here and reserved for final billing presentation.

Authorized Diagnostic Scope

• Diagnose and repair ECT sensor circuit.
• Replace thermostat and verify proper operating temperature.
• Re‑evaluate transmission noise and behavior after cooling system corrections.
• Perform EVAP smoke test and identify leak sources.
• Diagnose passenger presence system fault.
• Diagnose rear defrost circuit fault.

Customer Notes (Internal Tone)

• Transmission noise is likely secondary to ECT/temperature issues; internal transmission work is not assumed at this stage.
• Misfire is felt but not counted due to fallback strategy after code clearing.
• EVAP leaks affect emissions and check‑engine light, not drivability.
• PPS fault is safety‑relevant and will be addressed after primary drivability concerns.

Cooling / ECT

• ECT scan data matches IR gun within acceptable range.
• Engine reaches and maintains proper operating temperature.
• No active P0115 or P0128 after drive cycle.

Transmission

• No buzz/whirr in Drive/Reverse after ECT/thermostat repair.
• Shifts feel consistent and appropriate for load.
• No new TCM or gear ratio codes present.

EVAP

• No visible smoke leaks after repair.
• No active EVAP leak codes after drive cycle.

PPS / Rear Defrost

• PPS reports correct status with and without passenger.
• No active PPS or SDM communication codes.
• Rear defrost operates correctly; no short/open codes present.