OBD-II Diagnostic Trouble Code
P0400 — Exhaust Gas Recirculation Flow Malfunction
P0400 means the ECM detected a general EGR flow fault: the measured or inferred EGR flow does not match what it commanded. On heavy diesels, flow is usually calculated from an EGR differential pressure sensor or venturi, EGR temperatures and valve position, so many different faults can set this code.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Exhaust Gas Recirculation Flow Malfunction
Code
P0400
Category
Powertrain
Subsystem
Emissions
Truck-relevant
Yes
What this means for truckers
Service within a few days on a Cummins X15, Detroit DD15, PACCAR MX-13. Performance is likely degraded. Continuing to drive without service risks additional component damage.
Before you replace parts — try this first
P0400 on the Emissions system is most often sticking or carboned-up egr valve — 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 and possible engine derate. If flow is too high, expect black smoke, lost power, higher soot loading and more frequent DPF regens on diesels, or rough idle and stalling on gas engines. If flow is too low, the truck often drives normally but NOx rises, which can lead to higher DEF use or NOx-related codes on SCR-equipped diesels.
Common causes
- CommonSticking or carboned-up EGR valve ($600–$2500)
- CommonFaulty EGR differential pressure sensor or plugged sensor lines ($200–$700)
- OccasionalRestricted or leaking EGR cooler ($1500–$5000)
- OccasionalLeaking EGR pipes, clamps or gaskets ($200–$900)
- OccasionalEGR actuator wiring or connector fault ($100–$500)
How to fix
Run the scan-tool EGR valve actuation test and compare commanded versus actual position
When to use: First step
Check EGR differential pressure readings at key-on engine-off and running, and clear or replace sensor lines
When to use: When the valve moves correctly
Remove and clean or replace the EGR valve
When to use: When the valve sticks or responds slowly
Pressure test or inspect the EGR cooler for plugging or coolant leaks
When to use: When EGR temperatures or flow stay wrong after the valve and sensor check out
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
- → 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)
- → P0430 — Catalyst System Efficiency Below Threshold (Bank 2)
Frequently Asked Questions
Can I drive with code P0400?
P0400 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 P0400 on a commercial truck?
Total repair cost ranges $100–$5000 depending on the root cause. The cheapest fix is usually the first one listed below — Sticking or carboned-up EGR valve — 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 P0400 fail a DOT inspection?
Yes — P0400 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 P0400 and related codes?
P0400 is closely related to: 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)). 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 P0400 is set without the companions, the issue is more isolated.
How do I prevent P0400 from coming back after a repair?
For P0400 (Exhaust Gas Recirculation Flow Malfunction), 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.