I know that you have to earth spark plugs if you want to observe the spark.
If you have a 'cassette' type coil is it ok to use a single spark plug to test each cylinder's coil in turn, earthing only the single spark plug, or will this cause damage to the coil / ECU?
The ECU could be damaged if any HT Leads are not connected to earthed spark plugs, some ECUs can handle an open HT Lead and others can't, I don't have a list of those that can and can't
