Relay coil suppression with unknown switching tolerance
Relay coil suppression is typically a diode placed across the coil with the cathode connected to the positive side and the anode connected to the negative/ground side. A diode from the 1N400X series would be satisfactory.

When the ECM removes power from the relay coil, the collapsing magnetic field of the coil produces a reverse voltage across its terminals. The diode then conducts, shorting out this reverse polarity transient and therefore protecting the ECM. If the ECM has built in protection, the extra diode will cause no harm.