A 2014 Audi Q5 arrives with a P023600 code, and the technician's only note on the job card is "motor startar men går dålig" (engine starts but runs poorly). That single boost pressure code on a diesel engine can send a tech down a long path of testing components that are actually working fine.
Start with the G31 sensor
When our diagnostic platform analysed this specific job, it ranked a defective G31 boost pressure sensor as the most likely cause at 35 percent probability with a medium severity. This MAP sensor is responsible for measuring intake manifold pressure. On diesel engines like this one, soot contamination on the measuring port is a frequent issue.
If the sensor is dirty or failing, it feeds irregular or implausible data to the ECM. The ECM then detects a mismatch between the expected boost, calculated from atmospheric pressure, engine load and RPM, and the reported actual boost.
The diagnostic steps for this are non-invasive. The tech should check live data from the G31 sensor and compare it against the atmospheric pressure sensor with the engine turned off. A visual inspection of the electrical connector and a physical check to see if the measuring port needs cleaning will confirm the fault before replacing any parts.
Pressure test the charge air system
Ranked closely behind the sensor at a 30 percent likelihood is a leak in the charge air system. A cracked hose, a weeping intercooler, or a loose clamp allows compressed air from the turbo to escape before it reaches the combustion chamber.
The ECM reads this low boost and throws the P023600 code. Because less air is entering the engine, the vehicle runs a rich fuel mixture, which explains the poor running condition and often results in black or grey smoke from the exhaust.
A visual inspection of all charge air hoses and connections is the first step. If nothing is obvious, a pressure test using a smoke machine on the charge air system will expose hidden leaks quickly.
Testing the VNT actuator and N75 valve
If the intake system holds pressure, the diagnostic path moves to the turbo regulating system, which ranked at 20 percent probability. This Audi uses a variable nozzle turbine where adjustable vanes control the exhaust flow to manage boost. The vanes are moved by a vacuum actuator, regulated by an N75 or N280 valve.
Soot can cause the actuator to bind, or the vacuum lines themselves can fail. If the N75 valve is defective, the turbo cannot build or regulate pressure correctly. The required tests are a visual check of the vacuum lines and the physical movement of the actuator, followed by a functional scan tool test and a resistance measurement of the N75 or N280 valve.
Do not skip the air filter
The final cause, sitting at a 10 percent probability with low severity, is a restricted intake. A heavily clogged air filter, accumulated debris like leaves, or a collapsed intake hose creates a vacuum ahead of the turbocharger.
This restriction limits the air reaching the turbo, causing the G31 sensor to report out of range values under load. It takes only a few minutes to visually inspect the air filter and intake tract to rule this out.
What do you see in the bays?
This breakdown shows how one code branches into four distinct mechanical paths. If you work on these VAG vehicles regularly, do you find yourselves cleaning G31 sensors more often than replacing actuators?