Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Description & Operation
Powertrain Control Module (PCM), instrument cluster and Natural Gas Vehicle Module (NGVM) are connected to multiplex communications network through a pair of twisted wires or bus. Bus network is also referred to as Standard Corporate Protocol (SCP). Bus consists of a (+) circuit and a (-) circuit. Generic Electronic Module (GEM)/Central Timer Module (CTM), Remote Anti-Theft Personality (RAP) module/Central Security Module (CSM), Restraint Control Module (RCM) and 4-Wheel Anti-Lock Brake System (4WABS) module are connected to the International Standards Organization (ISO) diagnostic communications network through a single wire. This article is used to diagnose bus and ISO communications network circuit faults. For module locations, see COMPONENT LOCATIONS .
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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.