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Trouble Codes

The C-4 diagnostic system is programmed to flash a series of codes through the "CHECK ENGINE" light. After light becomes activated by the system, it is necessary to ground trouble code test lead, with engine running, so system can flash proper trouble code.

When "CHECK ENGINE" light comes on, a trouble in C-4 system is indicated. A series of codes are programmed to assist in diagnosis and correction of the problem. See TROUBLE CODE IDENTIFICATION  table for explanation of codes.

Codes are flashed by "CHECK ENGINE" light after the trouble code test lead is grounded. See Fig 1.

Codes will be flashed as follows: Light will flash 1, 2, 4 or 5 times to indicate first number of code; then after a short pause, it will flash 1, 2, 3, 4 or 5 more times to indicate second number of code.

For example, if problem is a shorted coolant sensor circuit, light will flash one time, pause, then flash 4 times (Code 14). After a long pause, signal will repeat itself 2 more times. If more than one trouble spot is detected, one code series will flash, then the other code series will flash.

TROUBLE CODE IDENTIFICATION

Code Problem
12 No Signal To ECM
13 (1) Oxygen Sensor Circuit
14 Shorted Coolant Sensor
15 Open Coolant Sensor Circuit
21 (2) Throttle Position Sensor Circuit
22 (3) Grounded Closed Throttle or WOT Switch Circuit
23 Open or Grounded Carburetor Mixture Control Solenoid Circuit
43 Throttle Position Sensor Adjustment
44 (4) Lean Oxygen Sensor Signal
45 Rich Oxygen Sensor Signal
51 (5) Faulty PROM
52 Faulty ECM
53 Faulty ECM
54 Faulty Mixture Control Solenoid or ECM
55 Faulty TPS (If Used) or ECM
(1) If Codes 13 and 41 appear at the same time, see Code 43.
(2) On 1979 3.8L and 1980 2.5L engine, problem is a grounded WOT switch circuit and an open throttle switch circuit at the same time. On 1979 3.8L and 2.5L engine, codes 21 and 22 will appear at the same time.
(3) 1979 3.8L V6 and 1980 2.5L 4-cylinder engines only.
(4) If Codes 44 and 55 appear at the same time, and "CHECK ENGINE" light is off, check for a faulty oxygen sensor.
(5) Check PROM for faulty installation.
Fig 1: Trouble Code Test Leads Locations
G09354748Courtesy of GENERAL MOTORS COMPANY
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