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DTC P1600 - Pcm & TCM Serial Communication Fault

  1. Turn ignition on. Using a voltmeter, measure voltage (backprobe) between ground and PCM harness connector "A", terminal No. 15 (Blue wire). If reading is zero volts, go to next step. If reading toggles between zero and 5 volts, check for an intermittent problem. See ADJUSTMENTS - 1.8L article. If idle speed and IAC duty are within specifications, go to step 3). If idle speed is high and IAC duty is low, or idle speed is low and IAC duty is high, go to next step. If idle speed and IAC duty are high, or idle speed and IAC duty are low, replace PCM. Retest system.
  2. Adjust idle speed and IAC duty. If adjustments can be made with idle air adjusting screw, go to next step. If adjustments cannot be made with idle air adjusting screw, go to step 4).
  3. If idle speed remains within specification as headlights are turned on, check for an intermittent problem. See appropriate TESTS W/O CODES article. If reading is about 5 volts, check for open in Blue wire. Repair as necessary. If wire is okay, replace Transmission Control Module (TCM). Retest system.
  4. Turn ignition off. Disconnect TCM harness connectors. Turn ignition on. Using a voltmeter, measure voltage (backprobe) between ground and PCM harness connector "A", terminal No. 15 (Blue wire). If reading is about 5 volts, replace TCM. If reading is not as specified, check Blue wire for poor connection or short to ground. Repair as necessary. If wire is okay, replace PCM. Retest system.
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