OBD-II Diagnostic Trouble Code
P0402 — Exhaust Gas Recirculation Flow Excessive Detected
P0402 means the ECM detected more EGR flow than expected, often when the valve should be closed or nearly closed. A valve stuck open or a leaking valve is the usual cause.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Exhaust Gas Recirculation Flow Excessive Detected
Code
P0402
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
P0402 on the Emissions system is most often egr valve stuck partly open from carbon buildup — 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
Rough idle, stalling, hesitation off idle, and black smoke on diesels from too much exhaust gas in the intake. Power may be low, soot loading and regen frequency can rise, and the engine may derate. Gas engines can stumble and stall when coming to a stop.
Common causes
- CommonEGR valve stuck partly open from carbon buildup ($400–$2000)
- OccasionalFaulty EGR actuator or position sensor reporting the wrong position ($600–$2500)
- OccasionalFaulty EGR differential pressure or flow sensor giving a false high reading ($200–$700)
- RareEGR control circuit shorted so the valve is always commanded on ($100–$500)
How to fix
Command the EGR valve closed and confirm the position reading and actual seating
When to use: First step
Remove and clean or replace the EGR valve
When to use: When the valve does not close fully
Check the EGR flow or differential pressure sensor at key-on engine-off for a sensible reading
When to use: When the valve closes properly
Check the control circuit for a short that keeps the valve energized
When to use: When the valve opens with the key on and no command
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
- → 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 P0402?
P0402 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 P0402 on a commercial truck?
Total repair cost ranges $100–$2500 depending on the root cause. The cheapest fix is usually the first one listed below — EGR valve stuck partly open from carbon buildup — 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 P0402 fail a DOT inspection?
Yes — P0402 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 P0402 and related codes?
P0402 is closely related to: P0400 (Exhaust Gas Recirculation Flow Malfunction), P0401 (Exhaust Gas Recirculation Flow Insufficient 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 P0402 is set without the companions, the issue is more isolated.
How do I prevent P0402 from coming back after a repair?
For P0402 (Exhaust Gas Recirculation Flow Excessive Detected), 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.