What it means
Exhaust Gas Recirculation Sensor "A" Circuit Low: the signal voltage is below the expected range. Without valid EGR feedback the ECU disables exhaust gas recirculation and may set further EGR codes.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
EGR sensor A reports valve position or another EGR measurement to the control module, depending on the vehicle's design. P0405 sets when the sensor A signal is lower than the expected electrical range, so the module cannot reliably use that feedback to control EGR flow.
Common causes
Most likely first.
- EGR position sensor failed
- Wiring or connector fault
- Connector pins corroded - high contact resistance
Symptoms
- Check engine light on
- Rough idle or hesitation
- Reduced engine performance
- Increased emissions
How to diagnose it
- Save freeze-frame data and compare EGR sensor A signal with the commanded valve position and other available EGR live data.
- With the ignition off, inspect the sensor A connector and harness for corrosion, loose pins, breaks, or chafing.
- Check the sensor's reference, ground, and signal circuits using the manufacturer's wiring information.
- Measure the sensor A signal under the conditions that set the code and check for an unintended short to ground or excessive connection resistance.
- If the circuit checks pass, test the EGR sensor A response through its operating range before replacing it.
Common fixes
- Clean or repair corroded, loose, or damaged sensor A connector terminals.
- Repair a signal wire shorted to ground or a damaged harness section.
- Restore a poor sensor ground or reference connection if testing identifies it.
- Replace EGR sensor A if its signal remains below range with a sound circuit.
Can I keep driving?
If the engine runs normally, a short, gentle trip is generally possible, but avoid driving if it stalls or loses power. The module may limit EGR operation, increasing emissions and potentially causing further EGR faults.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.