P0407
PowertrainExhaust Gas Recirculation Sensor B Circuit Low
Carbon-clogged EGR valves and tired position sensors are behind most of these, usually on a higher-mileage diesel that does a lot of short town runs. The ECU watches the voltage from the B position sensor inside the EGR valve, and when that signal drops too low, below the range the ECU expects, it logs P0407. Sometimes it's the sensor itself failing, sometimes it's a broken or shorted wire feeding it, and sometimes the valve has gummed up so badly the sensor can't read movement properly.
ⓘ Information only. This page provides general educational information about fault code P0407. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0407 mean?
P0407 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Exhaust Gas Recirculation Sensor B Circuit Low.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, frequently with no other obvious change to how the car drives
- • Lumpy or unsteady idle, sometimes hunting up and down or sitting higher than normal
- • A flat spot or hesitation pulling away from a standstill
- • Slightly heavier on fuel, the sort of thing you only spot over a tank or two
- • Puff of smoke under load on diesels if the valve is stuck and EGR flow is wrong
- • On some cars, the engine drops into limp mode and won't rev past a set point
Possible causes
Causes commonly associated with P0407, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. EGR position sensor (sensor B) failed or shorted internally, the most common cause once a car is past 80,000 miles
- 2. Carbon build-up jamming the valve so the sensor can't track its position. Classic on diesels driven mostly around town
- 3. Wiring to the sensor chafed, corroded or broken, often where the loom runs near hot exhaust components
- 4. Loose or carbon-fouled connector pins giving a poor signal
- 5. Signal wire shorted to earth, which drags the voltage down and triggers the low-circuit reading
- 6. Faulty EGR vacuum solenoid on older vacuum-operated systems
- 7. Internal ECU fault, but this is rare and worth ruling everything else out first
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull all the codes, not just P0407. Companions like P0401, P0405 or P0408 tell you whether you're chasing a sensor, a flow problem or both
- 2. Get eyes on the EGR valve, its connector and the loom running to it. Look for melted insulation, green corrosion on pins and chafe marks against the engine or bulkhead
- 3. With a multimeter on the sensor signal wire, check the voltage. A reading sat near zero usually means a dead sensor or a short to earth rather than a clogged valve
- 4. Confirm the 5-volt reference and the earth are both present at the connector with the key on and engine off. No reference voltage points at wiring, not the sensor
- 5. If your scan tool can command the valve, open it and watch the live data. A sensor that responds smoothly but reads erratically often just needs the valve cleaning; one that doesn't move at all means wiring or a dead sensor
Common questions about P0407
Should I buy a cheap EGR sensor or valve off eBay, or pay for genuine? +
For the sensor on its own a good aftermarket part is fine and saves you a fair bit. For the whole valve assembly I'd lean OEM or a reputable brand, especially on diesels. The cheap pattern valves often have sloppy sensor calibration and you end up with P0407 back on the dash within weeks, having wasted a job getting it in. If the valve is awkward to reach, the last thing you want is to do the labour twice for the sake of saving £40 on the part.
Can I keep driving with this code showing? +
Short trips, yes, the EGR system is about emissions rather than keeping the engine alive, so it won't strand you on the motorway. But a stuck or wrongly reported valve makes the car run rich, builds up more carbon and quietly eats your fuel economy. Diesels can drop into limp mode if the ECU isn't happy, which makes overtaking interesting. Get it looked at within a week or two rather than ignoring it for months.
Is this going to fail the MOT? +
The code by itself isn't tested, but if the engine management light is glowing when the tester looks at the dash, that's logged as a major defect and it's a fail straight away. On top of that, a faulty EGR can push the emissions readings out of limits, particularly the smoke test on a diesel. Sort the fault, clear the light and drive a few cycles to make sure it stays off before you book it in.
What's it likely to cost to put right? +
Depends entirely on what's actually wrong and where the valve lives. A wiring repair or connector clean can be £40 to £80 at an independent. A standalone position sensor runs roughly £100 to £200 fitted. A full EGR valve replacement is commonly £250 to £500 at a good independent, but it climbs into four figures on engines where it's tucked under the turbo or DPF, the VAG 1.6 TDI and similar diesels being the usual culprits for big labour bills. Main dealers will be dearer on labour, though genuine parts come with a longer warranty if you keep the car a while.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →