Ignition system provides 2 timing inputs to ECM, high resolution (360 pulses per camshaft revolution) and low resolution (8 pulses per camshaft revolution). ECM uses these 2 inputs to determine individual spark timing for each cylinder. Once ECM calculates ignition timing, timing signal will be sent to ignition coil driver on EST circuit. Each timing pulse received by ignition coil driver will trigger coil driver to operate ignition coil. EST signal voltage ranges from about .5 volt to 4.5 volts.

NOTE: Test numbers refer to numbers on diagnostic chart.

Code 42 will set if engine speed is less than 3000 RPM and voltage on EST circuit is less than about .5 volt. If engine starts at this point, code is intermittent.
This test checks for an EST signal from ECM to ignition coil driver.
Remaining tests check driver circuitry.
Because coil driver gets its power from coil, check ignition coil feed circuit to ignition coil for opens.
A Code 42 will set if high and low resolution circuits are shorted to each other. Before replacing any components, ensure these circuits are not shorted.

Fig 1: Code 42 Schematic (5.7L "F" Body) EST Circuit Grounded

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

Code 42 - Est Circuit Grounded (5.7L "F" Body)

Use the Manual With the Right Hardware

When to See a Mechanic

Rather have a pro handle this?

Safety Warning

Code 42 - Est Circuit Grounded (5.7L &quot;F&quot; Body)

Use the Manual With the Right Hardware

When to See a Mechanic

Rather have a pro handle this?

Code 42 - Est Circuit Grounded (5.7L "F" Body)