OBD-II Diagnostic Trouble Code
P0405 — Exhaust Gas Recirculation Sensor A Circuit Low
P0405 means the ECM sees the EGR sensor A signal stuck below its expected voltage range. Sensor A is usually the EGR valve position sensor, though on some engines it is a differential pressure sensor, so check the OEM definition.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Exhaust Gas Recirculation Sensor A Circuit Low
Code
P0405
Category
Powertrain
Subsystem
Emissions
Truck-relevant
Yes
What this means for truckers
P0405 (EGR Sensor A Low) — see existing context.
Before you replace parts — try this first
P0405 on the Emissions system is most often signal wire shorted to ground or open 5-volt reference — 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, the EGR system may be disabled, higher NOx, and possible derate on diesels. Drivability is often normal unless the EGR valve is also stuck.
Common causes
- CommonSignal wire shorted to ground or open 5-volt reference ($100–$500)
- CommonFailed EGR position or pressure sensor (often built into the valve) ($300–$2500)
- OccasionalCorroded connector at the EGR valve or sensor ($80–$300)
- OccasionalShared 5-volt reference pulled down by another sensor ($100–$600)
How to fix
Check for 5-volt reference and ground at the sensor connector
When to use: First step
Inspect and repair harness damage near the exhaust and EGR components
When to use: When reference is good but signal is low
Replace the sensor or EGR valve assembly if the sensor is integrated
When to use: When wiring and reference test good
Unplug other sensors on the shared reference to find a short
When to use: When several low codes set together
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 P0405?
P0405 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 P0405 on a commercial truck?
Total repair cost ranges $80–$2500 depending on the root cause. The cheapest fix is usually the first one listed below — Signal wire shorted to ground or open 5-volt reference — 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 P0405 fail a DOT inspection?
Yes — P0405 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 P0405 and related codes?
P0405 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 P0405 is set without the companions, the issue is more isolated.
How do I prevent P0405 from coming back after a repair?
For P0405 (Exhaust Gas Recirculation Sensor A Circuit Low), 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.