C1298 — PCM Class 2 Serial Data Link Malfunction

Causes

• Open, short, or high-resistance condition on the Class 2 serial data wire or associated connectors
• Poor battery voltage or weak/unstable charging system
• Corroded, loose, or damaged connector(s) at PCM or other network modules
• Faulty PCM or another module on the Class 2 bus causing bus contention
• Blown fuse or faulty relay supplying power to modules on the bus
• Aftermarket device (alarm, remote start, stereo) improperly connected to data or power circuits

Symptoms

• Multiple warning lights (check engine, ABS, traction control, or other module lamps) or network-related lamp illumination
• Intermittent or complete loss of communication with the PCM on a scan tool
• Multiple, apparently unrelated DTCs stored for different modules
• Erratic module behavior (intermittent features, communication timeouts)
• Possible drivability issues if PCM commands are missing or delayed

What to check

• Use a capable scan tool to read all stored codes from all modules; note any other U or C codes
• Verify and record battery voltage with key ON and engine OFF (should be within manufacturer spec)
• Inspect fuses and relays that feed the PCM and other network modules
• Visually inspect the PCM connector(s) and other module connectors for corrosion, bent pins, or damage
• Check for aftermarket devices connected to data, power, or ground circuits
• Backprobe the Class 2 data line and observe for activity with a scan tool, data bus monitor, or lab scope

Signal parameters

• Class 2 serial data should show digital data traffic when key is ON (modules exchanging messages)
• Data line must not be shorted to constant battery, to ground, or show continuous steady voltage
• Voltage/transitions and waveform characteristics should match manufacturer specifications — measure with scope or data bus monitor
• No excessive noise or repeated collisions on the bus; generally a single-wire serial traffic pattern when healthy
• Presence/absence of messages and error frames are diagnostic — compare to known-good vehicle or factory data

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