I replace my TCV unit this past fall because of this error code. I replaced it with a TCV from IPD I got the 'HD' version and got the new hoses as well at the same time. I tore the driver side of the car apart and serviced/cleaned everything up before installation of the new TCV unit.

Followed there installation instructions, setup and operation instructions. The p1237 code disappeared for the majority of the winter months. The P1237 code has come back again over the past month and it won't go away.

Was in contact with IPD and they had me go through a series of test/checks on the TCV unit. I sent them pictures as well. It has checked out perfect according to there specs.
So, the question remains what is causing this to happen?

I also still have the dreaded P1618 code as well. The cars codes have been cleared by two Volvo dealerships several times over the past year and the P1618 shows up about 1 month later after getting cleared out.

I've spent the past 18 months tracking/fixing error codes on this car and am getting tired of it now. Seriously thinking of getting rid of the car now.

For those who might be interested in the car, let me know.
Below is everything that I have done to it since I owned it:

Here is the laundry list of things I have done to the Car,

-New Brakes, Rotors, Pads, Calipers and SS Brake Lines. (Calipers are painted red with por15 caliper kit)
-New Heater core unit.
-New Shifter switch.
-Complete rework of the Interior of the Car.
-New Car Battery.
-New Plugs
-New Coils
-New Evap Hose
-New Silcone Turbo lines
-replaced all the vacuum lines
-New Oxygen Sensors (Front/Rear)
-New Cat. Converter Complete with pipes
-Remanufactured Throttle Body Unit from BBA (w/Lifetime Warranty)
-All New PCV System
-New Front Cam Seals.
-New Camshaft Position Sensor
-New Timing Belt
-New Serp Belt
-New outer Tie Rods(both)
-New Control Arms(both)
-New Sway bar Links(both)
-New Fuel Line on Car
-New Set of Tires for the Car(cooper cs5,January 2015)
-New Alternator
-New Spring Seats(front struts)
-New Sensor Switches on the exterior of the Transmission (Fall 2014)
-New Oil Cooler Gaskets
-New Turbo Gaskets
-New Turbo Control Valve with new hoses as well(IPD HD, October 2015)
-New Front Wheel Hubs (both, February 2015)
-ABS unit has been sent out to Midwest ABS. It was tested and is in perfect working condition. Was reinstalled. Midwest said, that unit should never fail on car. Basically 'bullet proof unit'!

Radiator was flushed in the fall 2014 complete. Brake fluid was completely flushed out when I did all new setup on the car in the fall of 2013. Oil changes done every 7500 miles with synthetics(Amsoil)and Mann oil Filters. Tires are rotated every 6000 miles. 4 wheel alignment done in spring of 2014 on vehicle. Have done 2 partial flushes on the transmission with Amsoil Synthetic Transmission Fluids, getting ready to do a third soon.

Also redid a lot of the wire tubing in the engine compartment as I have been working on things.