P2399 – Cylinder 8 Exhaust Gas Temperature Sensor Circuit

System: Powertrain | Standard: ISO/SAE Controlled | Fault type: Circuit | Location: Cylinder 8

Definition source: SAE J2012/J2012DA (industry standard)

DTC P2399 indicates the powertrain control system has detected a problem in the Cylinder 8 Exhaust Gas Temperature (EGT) sensor circuit. This is an electrical fault classification, meaning the issue is related to the sensor’s signal, its power/ground, wiring integrity, or connector condition rather than confirming a mechanical exhaust problem by itself. Because EGT sensor locations, circuit design, and enabling conditions vary by vehicle, the exact set conditions and diagnostic thresholds can differ. Always confirm the circuit type, pinout, and test specifications using the correct service information for the vehicle you are diagnosing.

What Does P2399 Mean?

P2399 – Cylinder 8 Exhaust Gas Temperature Sensor Circuit means the control module has identified an electrical circuit issue involving the exhaust gas temperature sensor associated with cylinder 8. Under SAE J2012 DTC structure conventions, the code points to a specific monitored circuit and a detected fault in that circuit’s operation. This does not, by itself, prove the sensor is failed; it indicates the expected electrical behavior of the cylinder 8 EGT sensor circuit was not met during the module’s monitoring checks, and further circuit-level testing is required to locate the root cause.

Quick Reference

• System: Powertrain
• Official meaning: Cylinder 8 Exhaust Gas Temperature Sensor Circuit
• Standard: ISO/SAE controlled
• Fault type: Circuit
• Severity: MIL illumination is possible; engine protection strategies or reduced performance may occur depending on how the system uses the EGT signal.

Symptoms

• MIL/Check Engine: Warning lamp illuminated and P2399 stored as a current or pending code.
• Reduced power: Limited torque or power output if the control module applies protective strategies based on missing/invalid EGT input.
• Regeneration issues: Aborted, inhibited, or repeated aftertreatment regeneration events on systems that rely on EGT feedback (varies by vehicle).
• Abnormal shifting: Transmission shift scheduling changes if powertrain torque management is affected (varies by vehicle).
• Fan operation: Cooling fan running more than expected if the module substitutes default values and commands additional cooling (varies by vehicle).
• Fuel economy change: Noticeable change in consumption if the engine is operating in a fallback strategy.
• Intermittent behavior: Symptoms that appear with vibration, heat soak, or during certain load conditions due to an unstable electrical connection.

Common Causes

• Connector issues: Loose, unplugged, mis-seated, or damaged connector at the Cylinder 8 exhaust gas temperature (EGT) sensor.
• Terminal problems: Corroded terminals, moisture intrusion, backed-out pins, poor pin tension, or fretting that creates unstable circuit continuity.
• Harness damage: Chafed, cut, melted, or pinched wiring near hot exhaust components, sharp edges, or moving parts.
• Open circuit: Broken conductor inside the insulation, fractured splice, or internal open within the sensor or pigtail.
• Short to ground: Signal or reference circuit rubbed through and contacting engine ground or exhaust heat shields.
• Short to power: Signal circuit contacting a power feed due to harness damage or incorrect routing/repairs.
• Power/feed fault: Missing sensor supply (varies by vehicle) due to a shared fuse, relay, or feed issue affecting the EGT circuit.
• Ground circuit fault: High resistance or open ground path (where applicable) from corrosion, loose fasteners, or damaged ground wiring.
• Control module circuit issue: Rarely, an internal fault or pin/terminal damage at the control module connector affecting the EGT input/output circuit.

Diagnosis Steps

Tools typically needed include a scan tool capable of reading freeze-frame data and live data, a digital multimeter, and service information for connector pinouts and circuit descriptions. Depending on vehicle design, a back-probe kit, terminal test tools, and a wiring repair kit are helpful. Use safe access equipment for under-vehicle inspection and follow hot-exhaust precautions.

1. Confirm the code and capture data: Verify P2399 is present. Record freeze-frame data and note when the fault set (load, temperature, speed). Check for additional DTCs that could indicate shared power/ground or harness issues.
2. Review service information: Identify the Cylinder 8 EGT sensor location, connector views, wire colors, pin functions, and whether the circuit uses a dedicated supply/ground or a reference/return strategy (varies by vehicle). Use the OEM test procedure sequence if provided.
3. Visual inspection (sensor and harness): With the ignition off, inspect the EGT sensor body and pigtail, then follow the harness as far as practical. Look for melted insulation, chafing, pinching, abrasion points, and evidence of contact with exhaust components.
4. Connector and terminal check: Disconnect the sensor connector and inspect for bent pins, corrosion, moisture, spread terminals, or backing-out. Verify connector locks and strain relief are intact. Repair terminal fitment issues before deeper electrical testing.
5. Wiggle test while monitoring: Reconnect as needed and run the engine/KOEO as appropriate. Monitor the EGT-related live data parameter(s) and/or circuit status while gently wiggling the connector and harness at multiple points. An abrupt change indicates an intermittent wiring/terminal fault.
6. Check for opens with continuity testing: Key off and connectors disconnected, test continuity end-to-end for the relevant EGT signal, supply/reference, and ground/return circuits per service information. If continuity is lost or unstable, isolate the open by segmenting the harness and inspecting splices and flex points.
7. Check for shorts to ground and shorts to power: With connectors disconnected, test each circuit for unintended continuity to chassis ground and to battery power. If a short is found, inspect harness routing and areas near heat shields, brackets, and clamps; correct the rub-through point and restore insulation integrity.
8. Voltage-drop testing under load: If the circuit uses a power and ground path, perform voltage-drop tests on the feed and ground while the circuit is operating (method varies by vehicle). Excessive drop indicates high resistance from corrosion, poor crimps, damaged wires, or weak terminal contact.
9. Sensor plausibility check via scan tool behavior: Using live data logging, observe the EGT reading behavior during a controlled warm-up and steady operating condition. Look for erratic jumps, dropouts, or a signal that does not respond in a reasonable manner compared to operating conditions. If the circuit tests good electrically but data remains implausible, follow service information to confirm whether the sensor itself is at fault.
10. Module-side inspection and pin fit: If wiring and sensor-side checks pass, inspect the control module connector for water intrusion, pin damage, or poor pin tension on the relevant circuits. Verify harness strain relief and connector seating. Avoid condemning a module until all external circuit faults are eliminated.
11. Clear codes and perform a verification drive: After repairs, clear DTCs and run the enable conditions (varies by vehicle). Recheck for pending codes and confirm the fault does not return. Review the post-repair log to ensure the EGT signal is stable and consistent.

Professional tip: Prioritize circuit integrity before replacing parts. Many EGT-sensor circuit faults are caused by heat-related harness damage or poor terminal tension that only shows up with engine movement and vibration. Use live-data logging during a wiggle test and follow up with voltage-drop testing to catch high-resistance faults that continuity checks alone can miss.

Possible Fixes & Repair Costs

Repair costs for P2399 vary widely because the root cause can be as simple as a poor connection or as involved as harness repair or sensor replacement. Parts access, required testing time, and labor rates also differ by vehicle and installation location.

• Repair wiring damage: Restore rubbed-through, melted, pinched, or broken conductors in the Cylinder 8 exhaust gas temperature sensor circuit, using proper splices and heat protection as required.
• Service connectors and terminals: Clean contamination, correct poor pin fit, remove corrosion, and replace bent or backed-out terminals; ensure locks and seals are fully seated.
• Restore power and ground integrity: Repair opens/high resistance in power feeds, sensor grounds, or shared grounds identified during voltage-drop testing.
• Replace the exhaust gas temperature sensor: Only after confirming the circuit is intact and the sensor is the verified failed component.
• Replace/repair related harness sections: If damage is widespread (heat exposure near exhaust routing), replace the affected sub-harness or re-route and re-secure to prevent repeat failures.
• Update/repair control-module connections: Address terminal tension or connector faults at the module side if testing proves the issue is not in the sensor-side wiring.

Can I Still Drive With P2399?

You may be able to drive with P2399 if the vehicle feels normal, but it is best to treat it as a powertrain electrical fault that can affect exhaust temperature monitoring and related control strategies. If you notice reduced power, unstable idle, abnormal shifting, warning messages, burning smells, or any signs of overheating, stop driving and have it diagnosed. Do not continue driving if the vehicle enters limp mode, stalls, or displays brake/steering warnings, as safe operation may be compromised.

What Happens If You Ignore P2399?

Ignoring P2399 can lead to ongoing warning lights, repeated fault storage, and reduced effectiveness of exhaust-temperature-based protections or emissions controls (varies by vehicle). A persistent circuit fault may worsen if heat or vibration continues to damage wiring, potentially causing intermittent operation, additional diagnostic trouble codes, and higher repair effort later.

Last updated: February 16, 2026

Vehicles Commonly Affected by P2399

• V8-equipped vehicles: Platforms that identify sensors by cylinder bank/numbering may set cylinder-specific exhaust temperature circuit codes.
• Turbocharged applications: Vehicles using exhaust temperature monitoring for component protection may be more likely to detect circuit issues.
• Diesel applications: Systems with extensive exhaust aftertreatment monitoring can be sensitive to sensor circuit faults.
• High-heat exhaust layouts: Tight packaging near manifolds, turbo housings, or downpipes can stress wiring and connectors.
• Vehicles with underbody exposure: Road debris, water intrusion, and corrosion can affect connectors and harness runs.
• High-mileage vehicles: Long-term vibration and thermal cycling increase the likelihood of terminal tension loss and insulation breakdown.
• Recent exhaust or engine work: Harness misrouting, pinched wiring, or partially seated connectors after service can trigger a circuit DTC.

After completing repairs, verify the fix by clearing codes and performing a drive cycle while monitoring relevant sensor data to confirm P2399 does not return.