I thought I would share this as it may help others. For years our Avensis Verso 2.0 D4-D has suffered with the dreaded 1251 fault code problem. Basically at random intervals (mostly many months apart) the vehicle suddenly goes into limp home mode (sudden loss of power but engine still running with warning light on). In our case pulling over, turning the ignition off and then on again got it out of limp home mode and all was OK for another few months. Toyota dealer said new turbo was needed. Forums seemed to suggest that cleaning the EGR valve was the solution. We just lived with it, although we had a few scary moments (actually downright dangerous!). One day, after a service, the mechanic said it seemed to lack power and sure enough it did. It was driveable but very flat. He suspected a MAF fault. I decided to have a go at fixing it myself. Here is the sequence I went through:
I took out the MAF sensor and cleaned it (it was quite dirty). Refitted it and the engine seemed smoother but no extra power.
Order new MAF on ebay. Fitted it. No difference.
Removed the EGR valve and cleaned it (all I had to hand was eco-friendly bicycle degreaser and an old toothbrush - worked a treat) lubricated the actuator with GT85 and manually worked the valve, rotating it at the same time to ensure it was clean all the way round.
Refitted EGR - again seemed to run smoother but still no extra power.
Checked turbo variable vane actuator linkage - this was fine and the vanes could be moved freely throughout their range with no sticking - squirted GT85 around the linkage and worked it back and forth just to be sure.
The actuator rod comes out of a small flying saucer shaped 'thing' to which a vacuum source is connected. Without vacuum the rod is extended and as vacuum is applied so the rod is drawn up into the 'thing' which in turn changes the pitch of the variable vanes in the turbo (or so it seems). I followed the vacuum tubing from the actuator back to the turbo vacuum valve (E-VRV). This is the device that modulates the vacuum supplied to the actuator.
Next I bypassed the E-VRV by connecting its vacuum supply tubing directly to the output tubing and then took the car for a spin round the block (cautiously as I'm not sure if this step is actually a good idea or not). It was immediately clear that the turbo was now doing what it should although as I sped up the engine fault light came on - but I suspected this would probably happen. To me this proved that the E-VRV was faulty, so I ordered a replacement (ebay)
Fitted the replacement and the vehicle is now driving better than it ever has. It is like having a new car. We have had no 1251 errors since. No more dropping into limp home.
I suspect that the E-VRV must develop some sort of intermittent fault and get temporarily stuck every now an then. Anyway if it hadn't failed completely I probably would not have found the true source of the 1251 problem on my particular vehicle. I hope this post helps someone else.