On these vehicles, if the exhaust warning light comes on and engine performance is poor, the following error codes may also be recorded.
P1550, synonymous with “boost pressure out of tolerance”.
P1556, synonymous with “boost pressure below lower tolerance limit”.
P1557, synonymous with “boost pressure above upper tolerance limit”.
As preliminary analysis, check the turbocharger’s blow-off valve and the subpressure line.
Another possibility is corrosion on the VTG turbocharger’s regulator linkage (variable turbine geometry). This may cause the linkage to jam sporadically, or even lock-up completely, so that the variable guide blades respond to the current load situation too late, or not at all. If a visual check indicates that the linkage is very corroded you should use mega macs to test the modulators. The linkage must be able to move freely and smoothly throughout its range, from one end position to the other, without jerking or stuttering.
You can restore the linkage’s freedom of movement by treating it with rust remover followed by spray-on grease. In intractable cases you will have to replace the entire turbocharger. If you have to do this, please follow the disassembly and reassembly instructions from the vehicle manufacturer.
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