P0408Generic code

P0408 Code: Exhaust Gas Recirculation Sensor B Circuit High

The P0408 code means the signal from EGR valve position sensor B has gone above its maximum value. The main effect is that part of the EGR system is switched off. An open circuit, a loose connector or a broken sensor ground is the usual cause.

Severity
Medium
Can I keep driving?
Yes, no immediate risk
Estimated cost
$60 to $500
Difficulty
Medium
System
Emissions Control
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0408 code means

Some engines, mostly diesels, have two EGR valves, for example a high pressure one and a low pressure one, or a single valve with two sensors. Sensor B reports the position of the second valve or of the second track.

The sensor gets a 5 volt reference and a ground from the computer and returns a signal between roughly 0.5 and 4.5 volts. The P0408 code sets when that signal is too high. That usually means an open circuit, such as a loose connector or a cut wire, or an interrupted sensor ground.

If the computer cannot verify position B, it switches that part of the EGR system off. On modern diesels with low pressure EGR, that can affect particulate filter regeneration and NOx emissions.

To confirm it, measure the continuity of each wire between the component connector and the computer connector with the key off. It should be under 1 ohm, and an infinite reading means that wire is open. If the wiring is good, measure the component itself, because infinite resistance confirms it is open inside.

P0408 code symptoms

  • Check engine light on.
  • Part of the EGR system switched off.
  • Incomplete emissions monitor, so the vehicle cannot pass inspection.
  • Loss of power or rough running.

Likely causes of the P0408 code

  1. Loose or broken connector. This is the most frequent cause.
  2. Signal or ground wire cut. The signal rises up to the reference.
  3. Sensor open inside, with infinite resistance.
  4. Short to 5 volts or 12 volts from rubbing against another wire.

How to diagnose the P0408 code step by step

  1. Read the sensor value on the scan tool and compare it with what you expect.
  2. Inspect the connector for corrosion, water or loose pins.
  3. Check reference and ground with the key on: 5 volts, and less than 0.1 volt of drop on the ground.
  4. Measure the signal with the connector plugged in and see whether it changes with conditions.
  5. Do a wiggle test on the harness while watching the reading.
  6. Check continuity all the way to the computer.
  7. Identify which valve is valve B in the manual for your engine.
  8. Compare desired and real position of that valve on the scan tool.

How to fix the P0408 code

  • Repair the connector or the wiring.
  • Replace the sensor if testing confirms it.
  • Correct the real condition if there is one, such as a leak or a blockage.
  • Clear the codes and complete a drive cycle so the monitor can run. If the P0408 code stays away, the repair worked.

Technical reference data

  • 5 volt reference, ground, and a signal of 0.5 to 4.5 volts.
  • These are approximate values, so check the manufacturer data for your engine.

Brand-specific notes

This code is common on Euro 5 and Euro 6 diesels with high and low pressure EGR. Identify which valve is B before you start testing.

P0408 on Volkswagen

Common on Volkswagen diesels with high and low pressure EGR.

P0408 on Ford

Common on Ford diesels with high and low pressure EGR.

P0408 on Renault

Common on Renault diesels with high and low pressure EGR.

P0408 on Mercedes

Common on Mercedes diesels with high and low pressure EGR.

Frequently asked questions about P0408

Can I drive with a P0408 code?
Yes. In general there is no immediate risk, and the problem is passing the emissions inspection.
Do I have to replace the sensor?
Only if testing confirms it. The connector and the wiring fail just as often as the sensor does.
Will I pass an inspection with this code?
No. The check engine light being on, or an incomplete monitor, keeps you from passing.
How much does it cost to fix P0408?
Roughly $60 to $500, depending on whether it is wiring, a connector or the valve assembly.
What does the P0408 code mean?
It is a generic OBD2 code for a high input in the EGR position sensor B circuit. The signal rose above its maximum value.
Will a P0408 code clear itself?
Only if the cause was a one-time glitch. An open wire or a failed sensor will bring the code back.
What is the most common cause of P0408?
A loose or broken connector, followed by a cut signal or ground wire. A sensor that is open inside is the next suspect.

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