What keeps the relay closed? Is there a latching circuit or is it the pressure sensor? Because it seems to me that this is where the problem is.

A couple of things I would look into:

• What’s on the heatsink? A transistor driving the relay
• What happens if you bypass the sensor and manually affect the circuit driving the relay?
• Going with the theory, that the relay is driven by a transistor, does said transistor have a diode across it?

I find the transistor being blown by the back EMF from the relay much more plausible, than a random resistor sliding out of spec.