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Engine Malfunction Test Procedure

  1. Enter diagnostics and record stored trouble codes. Begin diagnosis with lowest numbered code which is recorded. If codes "51" or "16" are shown, begin diagnosis with code "51", then proceed to code "16".
  2. If "intermittent failures" "13", "44" or "45" are displayed, use diagnostic chart for corresponding "hard failure" code. If "intermittent failure" code "33" appears, refer to diagnostic chart 33A.
  3. Code "25" indicates that ECM has detected a malfunction in the MD circuitry. When code "25" is set, ECM will only allow 8-cylinder operation. Refer to Performance Chart No. 16 for diagnosis of this code.
  4. Code "51" (if detectable by ECM) indicates faulty PROM installation. Refer to PROM removal and installation in this article for diagnosis of this code.
  5. Code "62" indicates that cruise control was engaged and vehicle speed exceeded 80 MPH. ECM allows operation of cruise control between 20 and 80 MPH. Clear code and road test vehicle.
  6. Code "64" indicates that cruise control was engaged and vehicle acceleration exceeded preset rate which was programmed into ECM. This could be caused by icy or wet pavement. Clear code and road test vehicle.
  7. Code "65" indicates that cruise control was engaged and coolant exceeded maximum temperature. Check cooling system, clear codes and road test vehicle.
  8. Code "66" indicates that cruise control was engaged and engine speed exceeded maximum allowable limit. This code can be caused by removing engine load (transmission in neutral) when cruise control is engaged and operating. Clear codes and road test vehicle.
    NOTE: After diagnosing trouble codes; switch tests, engine data displays and output cycling tests can be used to isolate "intermittent failures". DO NOT perform any adjustment or repairs on any component until malfunction has been positively located.
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