P0409: Exhaust Gas Recirculation Sensor A Circuit
What to do
Usually driveable. If the light comes and goes, mention that — an intermittent fault is diagnosed differently from a permanent one.
What this code means
A general circuit fault on the primary EGR position sensor, raised where the module can tell the circuit is wrong without resolving it into a high or low reading. Intermittent connections produce this more often than hard failures — a signal that drops out momentarily may never sit long enough at an implausible level to set the more specific code.
When the fault is intermittent, wiggling the harness and connector with live data displayed finds it far faster than static resistance measurements.
Likely causes
Start here
- Corroded or loose connector · easy
- Intermittent open in sensor wiring · moderate
- Faulty EGR position sensor · easy
Less common
- Damaged wiring harness near exhaust heat · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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