Well, probably:
- the EGR piping is clogged from just upstream of the vaccum valve, yet, if so, certainly downstream of the vacuum valve, so the sensor is not sensing any exhaust gas when the EGR system program expect to.
- the sensor has failed.
- the vacuum controlled egr valve is failing to a closed, or, the possible tragic condition of being stuck open, so hot exhaust gas are always getting into the engine. This failure can be gradual for a bit, yet fail dramatically causing you to have serious drivability and potentially engine damaging condition.

You could remove the sensor and inspect it. The twisted pair black cable is durable and terminates in a connector. Follow it back. If you use a small probe, liek a screwdriver into that hole vacated by the now removed EGR temp sensor, and you find a mass of nasty yucky oily stuff, this means you may need to remove, at least, the EGR pipe section from air intake port (manifold) and the other end and clean it out.

If the pipe appears clean, usually because you have been running full synthetic engine oil and you drive the car sensibly so the engine oil does not accumulate moisture and stuff, then the sensor is faulty or the EGR vacuum valve remains closed or is not fully closing. The sundry EGR vacuum pump could also be in a failed state or you have air intake leaks preventing the EGR vaccum valve from opening (doubtful, as you do not list other codes).

Have you checked for all engine control codes on your 1994 940? What system do you have? Regina Rex or Bosch LH? The EGR piping is more of less the same. The Bosch EZK ignition controls the EGR, but on newer models I'm not so sure.

Do you also have PULSAIR or some other system that directs fresh air into the exhaust port (manifold)?

Fault 4-1-3 illuminates the CEL.

Welp, hope this helps give you diagnosis direction.

Questions?

Heartburn Case.
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Carburettor? Intake and exhaust manifold. Injection? Air intake and exhaust port.