The posts above will, I believe, point you in the right direction. I would add that this happened regularly on my 95 HSE (about once per month). I think this can particularly happen during the starting sequence if the battery is dropping volts (through age, for example) especially if, for example, the engine stalls or comes under load before the SRS test sequence has completed. The system measures for a voltage across a check resistor to validate that the circuit is complete and if the supply volts are on the low side, for any reason, then the tst may fail. I believe there is an after market mod that can be performed to change the resistor.
In my case, I changed the battery and the the condition now only happens once in a blue moon.
The only way I found to clear the fault is through a diagnostic tool such as Rovacom lite or Testbook, it may be worth trying the other methods suggested.