What's the difference between P0405 and P0406?

They're the low and high ends of the same EGR 'Sensor A' position-feedback circuit. P0405 sets when the feedback voltage is too low — usually a short to ground. P0406 sets when it's too high — usually an open circuit or a short to the reference voltage. Both mean the PCM has lost a trustworthy reading of where the EGR valve actually is, even though the valve may be moving fine. The direction tells you whether to hunt for a break/short-high (P0406) or a short-to-ground (P0405).

Does P0406 mean I need a new EGR valve?

Not automatically. On most gasoline engines the position sensor is built into the EGR valve, so a genuinely failed sensor does mean a valve replacement — but the most common causes of P0406 are a corroded connector or an open wire, both of which are far cheaper to fix. Some platforms also split EGR feedback into a separate temperature/feedback sensor that can be replaced on its own. Check the connector and wiring first; only condemn the valve once the harness tests clean.

Can I keep driving with P0406?

Usually yes. When the PCM loses EGR position feedback it defaults to holding the valve closed, so the engine keeps running — you'll mostly see a small fuel-economy drop and an emissions-test failure. There's no immediate driveability danger on most vehicles. The exception is some European platforms where EGR is tied into intake throttle control, where lost feedback can cause more noticeable hesitation. Plan the repair within a few weeks rather than treating it as urgent.

How much does it cost to fix P0406?

It spans a wide range because the causes do. Reseating or cleaning a connector can be free to under $50. A pigtail or harness repair is $100-400. If the integral position sensor has failed and the whole EGR valve needs replacing, expect $300-800 on most platforms. If your engine uses a standalone EGR temperature/feedback sensor instead, that's cheaper at roughly $90-420. Diagnosing carefully before buying parts is what keeps this from becoming an unnecessary valve replacement.

OBD-II trouble code

P0406: Exhaust Gas Recirculation Sensor 'A' Circuit High

The EGR position-feedback signal is reading too high — pinned near the top of its range. This is the high-side mirror of P0405: the valve may be moving correctly, but the 'A' sensor reporting its position has gone open or shorted high, so the PCM no longer trusts the feedback.

Quick facts

System: Powertrain
Category: Emissions / Other
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $50 – $800
DIY difficulty: Intermediate DIY

What does P0406 mean?

P0406 is the high-input member of the EGR 'Sensor A' pair, and it's the direct counterpart to P0405. Both codes are about the position-feedback side of the EGR valve, not the valve's ability to move. Modern electric EGR valves carry a position sensor — usually a potentiometer-style element — that tells the PCM exactly how far the valve has opened. P0405 sets when that feedback voltage drops below the minimum; P0406 sets when it climbs above the maximum the PCM expects. The valve itself can be doing its job perfectly while the sensor circuit reports an impossible position.

The direction of the fault narrows the diagnosis. A high-pinned feedback signal usually means an open circuit — a broken signal-return wire, a corroded connector that's lost continuity, or a sensor element that has failed open. It can also mean the signal wire has shorted to the 5V reference or to another high-voltage circuit nearby. That's the same family of causes as a typical 'circuit high' sensor code: look for a break or a short-to-high rather than a short-to-ground.

It's worth understanding that 'EGR Sensor A/B' means different things across manufacturers. On most gasoline platforms, Sensor A is the valve's integrated position potentiometer, which is why this page's primary repair points at the EGR valve. On some systems the EGR feedback is split across a position channel and a separate temperature/feedback element, which is why a dedicated EGR temperature sensor repair is also offered — on those platforms the 'sensor' that needs replacing may be a standalone part rather than the valve. Identifying which architecture your engine uses is the key first step before buying anything.

Like the rest of the EGR feedback family, P0406 rarely causes dramatic drivability problems because most PCMs default the EGR closed when they lose trustworthy position data. The practical costs are a small fuel-economy hit, a guaranteed emissions-test failure, and — on platforms where EGR ties into intake throttle control — occasional light-throttle hesitation.

Common causes

Open signal-return wire from the EGR position sensor to the PCM
Corroded EGR valve connector that has lost continuity on the feedback pin
Position sensor failed open internally (not separately serviceable on most valves)
Signal wire shorted to the 5V reference or another high-voltage circuit
Pushed-back or broken terminal in the EGR connector
On split-feedback platforms, a failed standalone EGR temperature/feedback sensor
EGR valve connector left unseated after recent service
Aftermarket EGR valve producing a feedback range outside spec

Symptoms

Check engine light on with P0406 stored
Mild reduction in fuel economy
Engine often runs normally — most PCMs default the EGR closed when feedback is lost
Light-throttle hesitation on some platforms
Failed emissions inspection
On platforms where EGR ties to intake throttle, possible secondary throttle behavior

Diagnostic steps

1.Confirm the EGR valve connector is fully seated and inspect the feedback pin for green corrosion, oil contamination, or a pushed-back terminal.
2.Pull all codes. P0406 alone is the position-feedback-only case. P0406 with P0405 or the P0403/P0404 circuit codes broadens the picture.
3.With key on, engine off, measure voltage at the position-signal pin. A high reading pinned near the 5V reference points at an open return or a short-to-high.
4.Back-probe the signal at the PCM connector. If it reads high at the valve but normal at the PCM, the harness has an open or short between the two.
5.Check the signal-return (ground) wire for continuity — an open return is the classic cause of a high-pinned feedback signal.
6.On split-feedback platforms, determine whether the 'A' sensor is the valve potentiometer or a standalone temperature/feedback sensor before deciding what to replace.
7.If wiring tests clean and the valve carries the sensor, replace the EGR valve. Command it through its range afterward and confirm feedback tracks correctly.

Repair cost

$50 – $800

Reseating an unplugged connector is free; a connector pigtail repair runs $100-250 and a harness repair $150-400 depending on access. If the position sensor is integral to the valve and has truly failed, full EGR valve replacement is $300-800 by platform. On split-feedback systems where a standalone EGR temperature/feedback sensor is the culprit, that sensor is cheaper — roughly $90-420 installed. European platforms with integrated intake-throttle EGR assemblies can exceed $1,000. The 10x spread is why connector and wiring checks come before condemning the valve.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.

Related codes

P0405 P0407 P0408 P0403 P0404 P0401