P047B
PowertrainExhaust Pressure Sensor "B" Range/Performance
On most cars this turns out to be a sensing tube clogged with carbon and condensation rather than a dead sensor. The exhaust pressure sensor 'B' feeds the ECU a reading of the back-pressure in the exhaust, usually around the DPF on a diesel. When that signal drifts outside the expected window or stops agreeing with the other pressure readings while you're driving, the ECU flags P047B. It's almost entirely a diesel issue, and it's tied up with how the DPF manages its soot.
ⓘ Information only. This page provides general educational information about fault code P047B. 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 P047B mean?
P047B is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Exhaust Pressure Sensor "B" Range/Performance.
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 the dash, sometimes with no other obvious sign
- • Down on power or flat throttle, occasionally dropping into limp mode
- • DPF regeneration failing to complete, so the car never seems to clear its soot warning
- • Fuel economy creeping up on the same commute you've always done
- • Rough idle or the odd vibration when sitting at the lights
- • Black or sometimes blue smoke under hard acceleration
Possible causes
Causes commonly associated with P047B, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Sensing tube blocked with carbon or moisture. This is the usual culprit, especially on cars that do mostly short town trips where the exhaust never gets hot enough to burn things off
- 2. Sensor itself worn out and putting out a sloppy or drifting voltage, common past 90,000 miles on hard-worked diesels
- 3. Corroded or chafed connector pins at the sensor plug, very common where the loom runs near hot exhaust components and road salt gets at it
- 4. Intermittent open or short in the wiring between sensor and ECU, often a chafe point rubbing through the insulation
- 5. A restricted exhaust, partially blocked DPF or cat, giving real back-pressure the ECU doesn't like
- 6. EGR playing up and skewing the pressure picture the sensor sees
- 7. Rarely the ECU itself or a calibration issue, but check everything else 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 the live data and look at the actual pressure reading at idle and on a light load. A healthy sensor moves smoothly with engine load. If it's pinned, dead, or jumping about, you've found your direction
- 2. Scan for the codes that travel with it, P047A, P047C and the P2452 to P2455 family. A wad of pressure codes together usually points at the tube or the sensor rather than wiring
- 3. Disconnect the sensing tube and check it's clear. Carbon and watery sludge block these readily, and blowing it through or replacing the tube is often the whole fix
- 4. Eyeball the connector and the loom. Look for green corrosion on the pins, pushed-back terminals, and any spot where the wiring rubs against metal. Wiggle-test it with the data live
- 5. Back-probe the connector for the 5V reference and a clean ground with the key on. No reference voltage points at wiring or ECU, not the sensor
- 6. If the tube's clear, wiring's solid and the supply is good but the reading is still rubbish, the sensor's had it
Common questions about P047B
If I clear it, will it just come back? +
If all you do is wipe the code, it'll be back within a few drive cycles because nothing's changed. The exception is a one-off glitch, maybe a damp connector that dried out, in which case it might stay gone. But if the sensing tube is gummed up or the sensor's tired, clearing it buys you nothing. Fix the cause, then clear it and watch whether it returns over a week of normal driving.
What's the risk if I just leave it? +
On a diesel the big worry is the DPF. The ECU leans on this pressure reading to know when and how to run a regeneration, so a duff signal can stop the DPF clearing its soot. Keep driving like that and the filter slowly chokes, and a blocked DPF is a far bigger bill than a sensor or a length of tube. You'll also see worse economy and possibly limp mode. Not catastrophic overnight, but it gets worse the longer it's left.
How quickly do I need to sort this? +
It's not a pull-over-now fault. The car will usually keep running. I'd want it looked at within a week or two though, mainly because of the knock-on effect on DPF regeneration. If you're already getting power loss or a DPF light alongside it, move it up the list. The longer a diesel goes without completing regens, the closer you get to a forced regen at a garage or a filter replacement.
Is it the sensor itself or the wiring and plumbing? +
More often it's the plumbing and the wiring than the sensor. The little sensing tube blocking with carbon and condensation is the number one cause, and corroded connector pins near the hot exhaust come a close second. The sensor does fail, particularly on higher-mileage diesels, but check the cheap stuff first. A sensor is roughly £50 to £150 fitted at an independent garage, whereas clearing a tube or repairing a connector might cost almost nothing in parts.
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 →