NOTE: Numbers in list refer to circled numbers in diagnostic flow chart.

After performing the power balance test, if one cylinder or non-companion cylinders had no RPM change, see Fig 1 . If 2 companion cylinders had no RPM change, see Fig 2 . Otherwise, see Fig 3 .
The J 26792 spark tester (ST-125) presents a more difficult load on the secondary ignition than a normal spark plug. If a miss, stumble or hesitation is being caused by a spark plug not firing, the spark tester should also not fire.
The companion spark plug shares the same ignition coil with the spark plug being tested (1-4, 2-5, 6-7, 3-8).
A suspected ignition system 'miss' may actually be a fuel system problem. Perform FUEL & AIR CONTROL SYSTEM DIAGNOSIS CHART 6C-1: FUEL SYSTEM CHECK in SELF-DIAGNOSTICS -- ALLANTE article, if it has not yet been performed.
If the no spark condition follows the suspected coil, that coil is faulty. Otherwise, the ignition module is the cause of no spark. This test could also be performed by substituting a known good coil for the one causing the no spark condition.
If the no spark condition follows the suspected coil, that coil is faulty. This test could also be performed by substituting a known good coil for the one causing the no spark condition.

Fig 1: Secondary Ignition Check Flow Chart (1 Of 3)

Courtesy of GENERAL MOTORS CORP.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Notes On Fault Tree