One of my mates had been having problems with his 2006 Vauxhall Vivaro van.  First of all, he was having trouble starting the engine, it would start then cut out repeatedly, before eventually running.  Once running, the engine had no power under 2000 rpm, but once over 2000 rpm it suddenly ran like normal.

I checked for air leaks, cleaned the EGR valve and checked the wastegate actuator, all seemed fine.  I disconnected each injector control connector in turn to see if one of them  caused less change on engine power than the others, but all were equal.  Finally I used my oscilloscope to probe each of the wires connected to the Mass Air Flow sensor (MAF).  If the sensor was working correctly, I would have expected the signal on at least one of the wires to alter when the engine was revved, however there was no change.  We decided to purchase a non-original replacement MAF sensor and give it a try.

The MAF sensor located at the rear of the engine.  To remove it, two nuts that hold the air filter case had to be removed along with the jubilee clip that fastens the MAF sensor to the inlet hose.

There are then two screws that fasten the MAF sensor to the air filter housing that need to be removed.

Once the MAF sensor had been replaced, the engine was started.  At first, there seemed to be no difference, but after taking the van for a drive the engine performance continued to improve until the engine was running as it should, presumably the ECU re-calibrated itself over a period of time.