P0405
Exhaust Gas Recirculation Sensor A Circuit LowP0405 is a generic OBD-II powertrain diagnostic trouble code: Exhaust Gas Recirculation Sensor A Circuit Low. It is logged by the engine control unit when the egr monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0405 means
P0405 — "Exhaust Gas Recirculation (EGR) Sensor \"A\" Circuit Low" — is stored when the PCM/ECM detects that the voltage signal from the primary EGR valve position sensor falls below the minimum threshold of the expected operating range. Modern EGR valves use one or more integrated potentiometric (variable-resistance) position sensors to report the actual pintle or disc position back to the ECM, enabling closed-loop control of exhaust gas flow into the intake. The ECM commands a target position, monitors the sensor feedback to verify the valve reached that position, and trims the command accordingly. On single-sensor EGR valves, this sensor is designated \"A\"; on dual-sensor valves (common on Toyota 1GD/2GD diesel engines, some Ford Puma diesels, and Mazda SH-VPTS applications), sensor \"A\" is the primary feedback element and sensor \"B\" is the secondary.
The sensor signal voltage typically operates across a 0.5–4.5V window (referenced to a 5V supply from the ECM), with the low end of travel corresponding to a fully-closed valve and the high end to a fully-open valve, or vice versa depending on the manufacturer's calibration. P0405 is triggered when the ECM reads a signal below approximately 0.3–0.5V — a condition consistent with a short to ground on the signal wire, an open sensor ground path, a broken signal wire, or an internally failed sensor that outputs near-zero voltage regardless of valve position.
P0405 does not necessarily imply the EGR valve itself is mechanically faulty; the valve may be moving correctly while the position feedback circuit has an electrical fault. However, heavy carbon buildup in the EGR passage can seize the pintle in a fixed position, and if that position is the low-voltage end of the sensor's range, P0405 will be set alongside a mechanical complaint. Diesel engines are particularly susceptible to carbon-related sensor faults due to high soot loads in the EGR stream.
Common causes
Most-frequently reported root causes when P0405 is logged.
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1
Failed EGR valve position sensor \"A\" — internal potentiometer wear or burnout causing the output to collapse to near-zero voltage.
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2
Short to ground on the sensor \"A\" signal wire — chafed insulation, pinched harness, or moisture intrusion grounding the signal conductor.
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3
Open circuit in the sensor ground path — a broken or corroded ground wire causing the sensor reference divider to pull the signal low.
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4
Missing or low 5V reference voltage from the ECM — a shared reference circuit fault (blown fuse, short-to-ground on a sibling sensor) collapsing reference and signal together.
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5
Corroded, loosely-seated, or water-damaged EGR sensor connector causing high resistance on the signal path.
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6
Heavy carbon or soot buildup mechanically locking the EGR pintle at or near the closed position, holding the sensor at the low end of its travel.
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7
EGR valve actuator failure (motor or solenoid) preventing valve movement while the sensor reports the stuck position.
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8
PCM input circuit fault on the EGR sensor \"A\" channel (uncommon — only after all wiring and sensor checks are clean).
Symptoms drivers notice
How to diagnose P0405
A typical diagnostic flow when this code is present.
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1
Connect an OBD-II scan tool, retrieve all stored codes, and review freeze-frame data; note whether related EGR flow or actuator codes accompany P0405.
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2
With the ignition on and engine off, monitor the live EGR sensor \"A\" voltage — it should read approximately 0.5–1.0V with the valve at rest (closed); a reading below 0.3V confirms a signal-low condition.
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3
Inspect the EGR sensor connector and wiring harness for corrosion, oil contamination, bent pins, and chafed insulation along the routing path.
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4
Verify 5V reference voltage at the sensor connector; if reference is absent or low, trace back to the shared ECM reference circuit before condemning the EGR sensor.
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5
Check the sensor ground circuit continuity from connector to chassis ground — high resistance or an open ground will pull the signal output low.
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6
Disconnect the sensor connector and measure the sensor \"A\" potentiometer resistance from signal pin to ground pin across the full valve travel range — erratic or infinite resistance confirms a failed sensor.
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7
Inspect the EGR valve body for heavy carbon buildup; if the valve cannot be moved by hand through its range, clean or replace the valve before replacing the sensor.
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8
If wiring, ground, reference voltage, and sensor resistance all test good, perform an ECM circuit input check; replace the ECM only after all external components are confirmed serviceable.
Vehicles where we've handled P0405
Platforms in our catalogue with confirmed P0405 coverage.
Related powertrain codes
- P0400 — Exhaust Gas Recirculation Flow Malfunction
- P0401 — Exhaust Gas Recirculation Flow Insufficient Detected
- P0402 — Exhaust Gas Recirculation Flow Excessive Detected
- P0403 — Exhaust Gas Recirculation Circuit Malfunction
- P0404 — Exhaust Gas Recirculation Circuit Range/Performance
- P0406 — Exhaust Gas Recirculation Sensor A Circuit High
Frequently asked questions
What is the difference between P0405 and P0406?
P0405 means the EGR position sensor \"A\" signal voltage is below the lower limit of its expected range — indicating a short to ground, open ground, or internally failed sensor outputting near zero. P0406 is the opposite: the signal voltage is above the upper limit — indicating a short to the reference voltage, an open return path, or a sensor outputting near-maximum. Both are sensor \"A\" circuit faults; \"A\" vs \"B\" refers to which sensor on the EGR valve, not the circuit direction.
Can carbon buildup cause P0405 without an electrical fault?
Yes. If heavy carbon deposits lock the EGR pintle in the fully-closed position, the sensor will report a sustained low-voltage reading that matches the closed-position output. The ECM, expecting to see the voltage change when it commands valve movement, interprets the fixed low reading as a circuit fault and sets P0405. Cleaning or replacing the carbonized valve resolves the code in this scenario.
Is the vehicle safe to drive with P0405 active?
The vehicle is typically drivable for short distances. The ECM usually defaults to a fixed EGR rate or disables EGR, which keeps the engine running but may cause rough idle, higher NOx emissions, and some fuel economy loss. Extended driving is not recommended as the underlying fault can cause progressive emissions system wear, particularly on diesel engines with high EGR duty cycles.
Do I need to replace the entire EGR valve for P0405, or just the sensor?
On many modern EGR valves, the position sensor is integrated into the valve body and cannot be replaced separately — the whole valve assembly must be replaced. On some older or modular designs, the sensor can be replaced independently. Consult the manufacturer's parts catalogue for your specific application before purchasing parts.
Disabling P0405 in software
RaceTune can permanently disable P0405 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
ECUs with a P0405 disable in our catalogue
Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.
- Bosch EDC17C74 verified 2 software versions
- Bosch EDC16C31 verified 1 software version
- Bosch EDC17C50 verified 1 software version
- Bosch EDC17CP44 verified 1 software version
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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