OBD-II Diagnostic Trouble Code
P0408 — Exhaust Gas Recirculation Sensor B Circuit High
P0408 means the ECM sees the EGR sensor B signal stuck above its expected voltage range. Sensor B's role varies by engine (second position sensor, temperature or pressure sensor), so confirm the OEM definition.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Exhaust Gas Recirculation Sensor B Circuit High
Code
P0408
Category
Powertrain
Subsystem
Emissions
Truck-relevant
Yes
What this means for truckers
P0408 (EGR Sensor B High) — secondary EGR sensor open.
Before you replace parts — try this first
P0408 on the Emissions system is most often open sensor ground or signal wire — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Check-engine light, possible limited EGR and derate on diesels, and otherwise normal driving in many cases.
Common causes
- CommonOpen sensor ground or signal wire ($80–$400)
- CommonFailed EGR sensor B ($150–$800)
- OccasionalSignal shorted to voltage ($100–$500)
- RareECM input fault ($1200–$3500)
How to fix
Identify sensor B and check its ground and connector pins
When to use: First step
Check the signal wire for a short to voltage
When to use: When the reading stays high unplugged
Replace sensor B
When to use: When wiring tests good
Test the ECM input per OEM procedure
When to use: After the sensor and wiring are cleared
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0400 — Exhaust Gas Recirculation Flow Malfunction
- → P0401 — Exhaust Gas Recirculation Flow Insufficient Detected
- → P0402 — Exhaust Gas Recirculation Flow Excessive Detected
- → P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
- → P0421 — Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Frequently Asked Questions
Can I drive with code P0408?
P0408 is severity 3 — plan service within a few days. Performance is likely degraded already (you may feel reduced power, harsher shifts, or slightly worse fuel economy). Continuing to drive risks additional damage to adjacent components.
How much does it cost to fix P0408 on a commercial truck?
Total repair cost ranges $80–$3500 depending on the root cause. The cheapest fix is usually the first one listed below — Open sensor ground or signal wire — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0408 fail a DOT inspection?
Yes — P0408 is an emissions-related code, and DOT inspectors check emissions compliance. An active emissions code with a check engine light can result in an out-of-service violation under FMCSA Part 393. The truck typically can't be moved until the code is cleared. Repeated emissions violations escalate to CSA scores.
What's the difference between P0408 and related codes?
P0408 is closely related to: P0400 (Exhaust Gas Recirculation Flow Malfunction), P0401 (Exhaust Gas Recirculation Flow Insufficient Detected), P0402 (Exhaust Gas Recirculation Flow Excessive Detected), P0420 (Catalyst System Efficiency Below Threshold (Bank 1)). These codes share the same subsystem (emissions) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0408 is set without the companions, the issue is more isolated.
How do I prevent P0408 from coming back after a repair?
For P0408 (Exhaust Gas Recirculation Sensor B Circuit High), prevention centers on the emissions system. Keep up with PM on the EGR system — clean the valve at 200K miles. Use OEM-spec air filters. Don't ignore minor codes; they often escalate into bigger emissions failures.