P0408 is an OBD-II trouble code that points to a problem in your vehicle’s Exhaust Gas Recirculation (EGR) system, specifically the “EGR Sensor B Circuit High.” In plain English, your engine computer is seeing higher-than-expected voltage from one of the EGR position or pressure sensors. This can trigger a check engine light, rough running, and failed emissions tests. Understanding what P0408 means, what usually causes it, and how a mechanic would diagnose it will help you decide whether you can handle it yourself or need a shop.
What Does P0408 Mean?
P0408 stands for “Exhaust Gas Recirculation Sensor B Circuit High.” The ECU monitors the EGR system using one or more sensors (often a position sensor or differential pressure sensor). When the signal on the “B” circuit stays above a calibrated voltage range, the ECU flags P0408.
In most vehicles, this code does not mean the EGR valve is definitely stuck open or closed. It usually means the ECU does not trust the feedback signal it’s getting. That can be due to wiring issues, a failed sensor, or sometimes a bad EGR valve assembly that contains the sensor.
Quick Reference
- Code: P0408 – EGR Sensor B Circuit High
- System: Exhaust Gas Recirculation (EGR)
- Severity: Low to moderate (emissions and drivability)
- Common causes: Faulty EGR sensor, wiring fault, EGR valve issues
- Typical fix: Repair wiring/connector or replace EGR sensor/valve
Real-World Example / Field Notes
In the shop, I see P0408 most often on higher-mileage vehicles where the EGR valve and sensor are built into one unit. A common scenario: the customer comes in with a check engine light but says the car “drives fine.” After scanning, I find P0408 stored, sometimes with no obvious drivability complaints. A quick visual inspection reveals a brittle connector or green corrosion in the terminals. Clean and repair the connector, clear the code, run a drive cycle, and the problem is gone. In other cases, the EGR valve position sensor is simply worn out and the fix is replacement.
Symptoms of P0408
- Check engine light on: The most consistent symptom; P0408 usually sets a MIL.
- Rough idle: If the EGR valve is affected, you may notice unstable or shaking idle.
- Hesitation or stumble: Light throttle acceleration can feel choppy or weak.
- Reduced fuel economy: Incorrect EGR operation can slightly lower MPG.
- Failed emissions test: An active P0408 will usually cause an automatic fail.
- Engine pinging (spark knock): In some cases, incorrect EGR flow can lead to light detonation under load.
- No noticeable drivability change: On some cars, the only clue is the warning light.
Common Causes of P0408
Most Common Causes
- Faulty EGR position/feedback sensor: Many modern EGR valves have an integrated position sensor. When the sensor fails internally, it can send a constant high-voltage signal, triggering P0408.
- Damaged or corroded wiring to the EGR sensor: Broken wires, high resistance, or short-to-voltage in the signal circuit can make the ECU see an artificially high reading.
- Poor electrical connection at the EGR connector: Loose pins, water intrusion, or corrosion at the plug are extremely common, especially on vehicles exposed to road salt.
- Failed EGR valve assembly: On units where the valve and sensor are one piece, a mechanical failure can sometimes affect the sensor output.
Less Common Causes
- ECU (PCM) internal fault: Rare, but a damaged driver or reference circuit inside the ECU can cause incorrect voltage readings.
- Wiring harness damage elsewhere: Chafed harnesses against brackets or exhaust components can short the signal wire to the 5V reference or battery voltage.
- Incorrect or low-quality replacement parts: Aftermarket EGR valves or sensors that are out of spec can cause persistent P0408 even after replacement.
- Previous wiring “repairs”: Poorly done splices, twisted wires, or wrong wire routing from earlier work can create intermittent high-voltage signals.
Diagnosis: Step-by-Step Guide
To properly diagnose P0408, you should have at least a quality scan tool (capable of reading live data and Mode $06), a digital multimeter, and basic hand tools. Access to a wiring diagram or service information for your specific vehicle is very helpful.
- Confirm the code and check for companions: Use a scan tool to verify P0408 and look for related EGR or sensor codes (like P0405–P0409). Multiple codes can point you toward a wiring or power/ground issue.
- Check freeze-frame data: Note engine RPM, load, coolant temperature, and speed when P0408 set. This helps you know if it happens at idle, cruise, or under load.
- Visual inspection of EGR valve and wiring: Locate the EGR valve and its sensor connector. Look for melted insulation, rubbed-through wires, oil contamination, or corrosion in the connector.
- Wiggle test: With the engine idling and live data up (EGR position or sensor “B” signal), gently move the harness and connector. If the reading jumps or the engine stumbles, you likely have a wiring or connector issue.
- Check reference voltage and ground: With a multimeter, verify that the sensor is getting the correct 5V reference and a solid ground. A missing ground can cause the signal to float high.
- Measure signal voltage: Backprobe the EGR sensor signal wire. Key on, engine off, you should see a voltage within the specified range (often around 0.5–1.0V at rest, depending on design). If it’s pegged near 5V or battery voltage, suspect a short-to-voltage or bad sensor.
- Command the EGR with a scan tool (if supported): Use bi-directional controls to open and close the EGR. Watch the sensor “B” reading. If the commanded change doesn’t match the feedback, the sensor or valve assembly is suspect.
- Check Mode $06 and readiness data: Some scan tools let you see EGR monitor test results. This can confirm whether the ECU is consistently failing the EGR monitor or if it’s intermittent.
- Continuity and short tests: If voltage checks don’t add up, disconnect both ECU and EGR sensor, then ohm-check the signal wire from end to end and check for shorts to power or ground.
- Substitute with a known-good part (if available): In some shops, swapping in a known-good EGR valve/sensor is the quickest way to confirm a bad component once wiring is ruled out.
Pro tip: Always test the wiring and power/ground circuits before condemning the EGR valve. Many P0408 repairs end up being a simple connector or harness fix, not an expensive valve replacement.
Possible Fixes & Repair Costs
Most P0408 repairs involve restoring a clean, accurate signal from the EGR sensor “B” circuit. Depending on what you find in diagnosis, the fix can be simple or more involved.
- Clean and tighten EGR connector: Remove corrosion, apply dielectric grease, and ensure pins fit snugly.
- Repair or replace damaged wiring: Fix chafed sections, broken wires, or poor splices using proper solder/heat-shrink or OEM-style connectors.
- Replace EGR valve/sensor assembly: If the sensor is integrated and testing shows it’s faulty, replace the entire unit.
- Replace standalone EGR sensor: On systems with a separate DPFE or EGR position sensor, swap the faulty sensor and clear codes.
- ECU repair or replacement (rare): Only after confirming wiring and components are good.
Typical repair costs for P0408 vary widely. A simple wiring or connector repair at a shop might run $100–$250. Replacing an EGR sensor or valve assembly usually ranges from $200–$600 parts and labor, depending on vehicle make, part price, and how hard it is to access. ECU-related repairs, while rare, can exceed $800–$1,200. Labor rates, OEM vs aftermarket parts, and regional pricing all affect the final cost.
Can I Still Drive With P0408?
In most cases, you can still drive with P0408 without immediate engine damage, especially if the car feels normal. However, you may experience poor fuel economy, rough running, or spark knock under load. More importantly, your vehicle will likely fail an emissions test while the code is active, and the check engine light can mask new problems that pop up later. Treat P0408 as a “fix soon” issue rather than an emergency, and avoid heavy towing or aggressive driving until it’s resolved.
What Happens If You Ignore P0408?
Ignoring P0408 long-term can lead to increased emissions, potential carbon buildup in the intake, and possible engine knock if EGR flow is affected. You’ll almost certainly fail inspections, and you may end up with additional codes as the ECU tries to compensate. Fixing it early usually keeps the repair simpler and cheaper.
Related Codes
- P0474 – Exhaust Pressure Sensor Intermittent
- P0473 – Exhaust Pressure Sensor High
- P0472 – Exhaust Pressure Sensor Low
- P0471 – Exhaust Pressure Sensor Range/Performance
- P0470 – Exhaust Pressure Sensor
- P0469 – EVAP Purge Flow Sensor Circuit Intermittent
- P0468 – EVAP Purge Flow Sensor Circuit High
- P0467 – EVAP Purge Flow Sensor Circuit Low
- P0466 – EVAP Purge Flow Sensor Circuit Range/Performance
- P0465 – EVAP Purge Flow Sensor Circuit
Key Takeaways
- P0408 means the ECU sees a high-voltage signal from the EGR sensor “B” circuit, not necessarily a stuck EGR valve.
- Most issues trace back to a bad sensor, corroded connector, or damaged wiring near the EGR valve.
- Symptoms range from just a check engine light to rough idle, hesitation, and poor fuel economy.
- Proper diagnosis with a scan tool and multimeter saves money by avoiding unnecessary parts replacement.
- Typical repairs run from around $100 for simple wiring fixes up to $600 or more for EGR valve replacement.
Vehicles Commonly Affected by P0408
P0408 shows up across many brands that use electronically controlled EGR systems. It’s commonly seen on Ford and Lincoln trucks and SUVs (especially with DPFE-style sensors), GM vehicles such as Chevrolet and GMC pickups and SUVs, and various Chrysler, Dodge, and Jeep models. Some Toyota, Honda, Hyundai, and Kia vehicles with integrated EGR valve/sensor assemblies can also log this code. It’s more frequent on higher-mileage cars, work trucks, and vehicles driven in harsh climates where connectors and wiring are exposed to salt and moisture.
FAQ
Can P0408 clear itself?
Yes, in some cases P0408 can clear itself if the underlying issue is intermittent, such as a loose connector that temporarily makes good contact. However, if the root cause isn’t fixed, the code usually comes back after a few drive cycles once the ECU reruns the EGR monitor.
Is P0408 caused by a bad EGR valve?
It can be, but not always. P0408 points to a high signal on the sensor “B” circuit, which might be inside the EGR valve assembly or in a separate sensor. Many times, the real culprit is wiring or a failing sensor rather than the mechanical valve itself. Testing is the only way to know for sure.
Can a dirty EGR passage cause P0408?
A clogged or dirty EGR passage usually causes flow-related codes like P0401 or P0402, not P0408. P0408 is an electrical signal issue. However, if carbon buildup affects the valve’s movement and the sensor’s ability to track position, it can indirectly contribute, especially on integrated units.
How serious is P0408 for my engine?
P0408 is generally moderate in seriousness. It won’t typically cause immediate engine failure, but it can lead to drivability issues, poor fuel economy, and higher emissions. If the EGR system isn’t working correctly, long-term operation could contribute to carbon buildup and occasional knock.
Can I fix P0408 myself at home?
If you’re comfortable with basic tools and electrical checks, you can often handle the initial inspection yourself: checking the connector, cleaning corrosion, and looking for damaged wiring. Replacing an EGR sensor or valve is DIY-friendly on many vehicles. Just be sure to follow a service manual, use quality parts, and clear the code with a scan tool after repairs.