My S40 giving a permanent P1650 error code for a long time. I thought it
was because of worn spark plugs (I found an description on internet
telling to do so) so I replaced them but nothing changed. I am scanning
error codes with obd softwares digimoto and proscan. Whenever I cleared
the error code it appears again if engine is running.

Digimoto says:
"P1650 PSP Switch out of self test range"

where Proscan says:
"P1650 Powertrain: Control Module Output B Circuit" (what does this mean?)

I queried the error code on google and it returned some information like
this:

"P1650 - Power Steering Pressure (PSP) Switch Malfunction In Key On,
Engine Off Self-Test, this DTC indicates the PSP input to the PCM is
high. In Key On, Engine Running Self-Test, this DTC indicates that the
PSP input did not change state. Steering wheel must be turned during Key
On, Engine Running Self-Test
PSP switch/shorting bar damaged
SIG RTN circuit open
PSP circuit open or shorted to SIGRTN
PCM damaged."

the description above quoted from a message board dedicated to owners of
ford focus .

"P1650 Quad Driver Module ''A'' Fault"
from a suzuki forum

"P1650 Driver 2 - Input High Voltage

P1650 Power Steering Pressure Switch Malfunction

P1650 Power Steering Pressure Switch Out Of Range"
from a volvo850 forum.

What shall I do?