What does it mean?
The module compares the readings of the A and B turbocharger outlet pressure sensors and has found that they no longer agree. On engines fitted with two of these sensors the software expects both to report almost the same value at key-on, when they should both see barometric pressure, and to move together as boost builds. A difference larger than the calibrated tolerance for a set time means at least one of the two is wrong, or that something between the two measuring points is changing the pressure. The module cannot tell on its own which sensor is honest, so it usually falls back to a conservative boost strategy until the disagreement is resolved. The important diagnostic point is that this code does not automatically condemn either sensor.
Symptoms
- MIL on, often together with boost performance or overboost codes
- Reduced power and a boost level that never reaches the usual figure
- Uneven or surging boost delivery under load
- Occasional limp mode on hard acceleration
- Two boost values in live data that clearly disagree, most obviously with the engine switched off
Likely causes
- One of the two sensors has drifted with age and no longer matches barometric pressure at key-on
- Blocked or partly restricted pressure port on one sensor, from oil mist, soot or condensate
- Boost leak between the two measuring points, so the pressures really are different
- High resistance in one sensor's signal or ground circuit shifting its reported value
- Connectors swapped after repair work, so each signal arrives at the wrong module input
- Failed sensor stuck at a fixed value while the other continues to work normally
- Water in one connector producing a small but steady offset
How to fix it
- Start with the engine off and the key on and compare both sensors with barometric pressure; the one that does not match ambient is the suspect
- Log both sensors together during a road test and look for the point where they diverge
- Remove each sensor and check its port and tip for oil, carbon and water
- Pressure-test the charge air pipework between the two measuring points to rule out a genuine leak
- Check the signal and ground circuits of the suspect sensor for voltage drop and corrosion
- Confirm the connectors are on the correct sensors, especially if work has recently been done in that area
- Replace only the sensor that is proven wrong, then verify both readings agree at key-on and through the boost range