Assuming the sensor was disturbed while removing the subframe, it may just be a legacy fault if the ignition was turned on with the sensor disconnected. Have you tried clearing the code to see if it comes back?
If it does come back it could be a bad connection. I'd have a check and make sure the connector is fully seated, maybe remove it and give it a squirt of WD40.
If that doesn't help then replace replace the sensor.
|