P0405

Powertrain
Generic (SAE)
Emissions

Severity

Diagnose soon

EGR is disabled while the feedback is untrusted, so combustion temperatures rise and pinging under load is possible. It affects emissions compliance and drivability but poses no immediate safety or rapid-damage risk.

Safe to drive: Driveable. Ease off under heavy load if you hear pinging, and have it repaired before an emissions test.

Standard definition

Exhaust Gas Recirculation Sensor "A" Circuit Low

In plain English

The voltage coming back from the EGR position or pressure sensor is lower than the computer considers possible. Something in that sensor circuit is shorted to ground, open on the reference side, or the sensor has failed. The computer usually shuts EGR off until it can trust the reading again.

The PCM supplies the sensor with a regulated reference voltage, commonly five volts, and a ground, and reads back a signal that varies with EGR valve position or with differential pressure across the metering orifice. The signal has a defined valid window with the bottom of that window set below the lowest reading the closed valve can legitimately produce. A reading below that floor - typically at or near zero volts - sets P0405. Sensor A designates the primary EGR feedback device for that vehicle. Depending on the system, that is either a valve position sensor or a differential pressure feedback sensor - check service data for which device your vehicle uses, and note that some manufacturers report DPFE and position sensor faults through their own enhanced codes rather than through P0405.

How the car detected it

The PCM samples the signal line continuously and compares against a fixed lower plausibility limit. The limit is deliberately set below any legitimate valve position, so the code indicates a circuit condition rather than an unusual but valid valve state. Most calibrations require the condition to persist for a period before setting the code, to avoid tripping on momentary noise.

A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.

Can I keep driving?

Safe to drive: Driveable. Ease off under heavy load if you hear pinging, and have it repaired before an emissions test.
Urgency: Diagnose soon
Will it pass emissions: No
If ignored: Higher combustion temperature and NOx output, which will fail an emissions inspection. Prolonged detonation under load causes real piston and ring damage. Related EGR codes will typically accumulate as the system runs without feedback.
Check engine light: Solid
Clearing the light: A completed drive cycle is normally required before the monitor re-runs and the light can clear.

Likely causes

These are the causes this code can have. For the order to work through them in, see what to check first.

#	Cause	How often	Component
1	EGR position sensor failed	Very common	—
2	Wiring or connector fault	Common	—
3	Connector pins corroded - high contact resistance	Occasional	—

Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.

What to check first

In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.

That the sensor is actually plugged in and the connector is latched
An unlatched connector after recent service produces this exact code and takes seconds to rule out.
Other codes on sensors sharing the same five-volt reference
One shorted sensor pulls the shared reference down and sets low-voltage codes on several unrelated sensors at once. If you see a cluster, the fault is the reference, not this sensor.
The live signal voltage with the key on and the engine off
Confirms whether the fault is present now. A reading pinned at zero volts is very different from a reading that is simply low but responding.
The connector for corrosion, water, and pushed-back pins
The EGR sensor sits in a hot, exposed location. Corroded terminals and heat-embrittled wiring are common and visible.
Pressure hoses on differential-pressure sensor designs
A collapsed or blocked hose can hold the sensor at its floor reading with nothing electrically wrong.

How a shop diagnoses this

Read all codes and note which sensors share the reference circuit
Tools OBD-II scan tool, wiring diagram
Record every stored and pending code before clearing. If several sensors report low voltage simultaneously, stop diagnosing this sensor and go find the shorted device or the damaged reference wire that all of them share.
Expected
Either an isolated P0405, or a cluster of low-voltage codes that redirects the diagnosis to the shared reference.
Measure the signal voltage at the sensor with the key on
Tools Digital multimeter, backprobe leads, scan tool
Backprobe the signal pin against a known-good ground with the connector seated. Compare against the value the scan tool is reporting. If the scan tool and the meter disagree, the problem is between the sensor and the PCM.
Expected
A low but non-zero closed-valve voltage. Exact values are vehicle-specific, so use service data. Zero volts indicates a short to ground or a missing reference.
Verify the reference voltage and the sensor ground
Tools Digital multimeter, backprobe leads
Measure the reference pin against chassis ground - it should sit at the regulated value, commonly five volts. Then measure the voltage drop on the sensor's ground pin with the circuit live. A missing reference or a high-resistance ground both drive the signal down.
Expected
Reference at its regulated value. Ground-side voltage drop small - a few tenths of a volt at most.
Disconnect the sensor, then jumper reference to signal to prove the harness
Tools Digital multimeter, backprobe leads, fused jumper wire, scan tool
With the sensor unplugged and the key on, read the signal pin at the harness side. If it pulls up toward reference voltage, the harness is intact and the sensor is internally shorted. If it sits at zero, do not stop there - many PCMs have no pull-up on this line and a floating pin reads zero on a good harness. Confirm by jumpering the reference pin to the signal pin at the harness connector and watching the scan tool value: if it swings to the top of its range, the signal wire and the PCM input are both good and the sensor is at fault. If it stays at zero, the signal wire is shorted to ground between the connector and the PCM.
Expected
With the reference jumpered to the signal pin, the PID goes to the top of its range on a healthy harness.
Test the signal wire for a short to ground
Tools Digital multimeter, wiring diagram
With both the sensor and the PCM connector disconnected, measure resistance from the signal wire to chassis ground. Then check continuity end to end. Pay attention to the harness section that runs near the exhaust or EGR tube.
Expected
Very high resistance to ground, and low-resistance continuity from end to end.
Exercise the valve and watch the sensor track
Tools Bidirectional scan tool or hand vacuum pump
If the circuit checks out electrically, command the EGR valve through its range with a scan tool, or apply vacuum to it manually, and watch the sensor reading move. A sensor that reads plausibly at rest but drops out mid-travel has a worn internal track.
Expected
Smooth, continuous voltage change through the travel with no dropouts or flat spots.
Wiggle test the harness with live data displayed
Tools Scan tool or multimeter
With the key on and the signal voltage on screen, flex the harness from the sensor back toward the loom. Concentrate on heat-exposed runs and any point where the loom passes a bracket edge.
Expected
A steady reading throughout. Dropouts localize the fault.
Clear codes and confirm the monitor completes
Tools Scan tool with readiness monitor status
After the repair, drive the vehicle until the EGR monitor runs. An emissions test will fail on incomplete monitors even with no codes stored, so confirm readiness rather than assuming.
Expected
EGR monitor complete, no stored or pending codes.

Possible repairs and what they cost

Repair	Only after you confirm	Typical cost	Who
Replace the EGR position or differential pressure sensor	With the sensor unplugged the signal pulls up toward reference voltage, or the PID swings to the top of its range when reference is jumpered to the signal pin - with reference and ground both verified good.	$120–$360	DIY
Repair a signal wire shorted to ground	With the sensor disconnected the signal stays at zero and the PID does not move when reference is jumpered to the signal pin, with low resistance measured from the signal wire to ground.	$120–$450	DIY
Repair an open or damaged five-volt reference circuit	Reference voltage missing or low at the connector, often with other sensors on the same reference also setting codes.	$130–$500	DIY
Clean or replace a corroded sensor connector	Terminal corrosion or water intrusion found, with the reading changing when the connector is disturbed.	$80–$280	DIY
Replace collapsed or blocked EGR pressure hoses	Hoses found hardened, split, or carbon-blocked on a differential-pressure system.	$70–$200	DIY
Repair or replace the PCM	Verified good sensor, verified harness with no shorts or opens, and the reference voltage absent at the PCM output itself.	$700–$2,000	Shop

The sensor itself is usually inexpensive. Labor depends heavily on where it sits - some are on the front of the manifold and reachable in minutes, others are behind the engine or under the intake. Wiring repairs are priced by diagnostic time, not parts.

Common mistakes with this code

Replacing the sensor before checking that the five-volt reference is actually present at the connector.
Missing that several sensors are reporting low at once, which means the shared reference is the fault, not this sensor.
Reading zero volts on a disconnected signal pin and concluding the wire is shorted to ground, when many PCM inputs have no pull-up and read zero on a perfectly good harness - the jumper test from reference to signal is what actually separates a failed sensor from a shorted wire.
Overlooking a high-resistance sensor ground, which drags the signal down while every other check looks normal.
On differential-pressure systems, ignoring collapsed or blocked hoses that mechanically pin the sensor at its floor.
Fitting a low-quality replacement sensor and getting the same code back within weeks.

Components involved

EGR Position Sensor
EGR Sensor Wiring

Sources

Definition: Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
Diagnostic guidance: Written and reviewed in-house. See our editorial policy.
Review status: Reviewed

Found something wrong? Tell us — we publish corrections.

That the sensor is actually plugged in and the connector is latched

Other codes on sensors sharing the same five-volt reference

The live signal voltage with the key on and the engine off

The connector for corrosion, water, and pushed-back pins

Pressure hoses on differential-pressure sensor designs

Read all codes and note which sensors share the reference circuit

Measure the signal voltage at the sensor with the key on

Verify the reference voltage and the sensor ground

Disconnect the sensor, then jumper reference to signal to prove the harness

Test the signal wire for a short to ground

Exercise the valve and watch the sensor track

Wiggle test the harness with live data displayed

Clear codes and confirm the monitor completes