Diagnostic Trouble Codes

RG7774-UN-16MAR98

Diagnostic Connector

TO READ TROUBLE CODES: Short pin A to pin D. The “Service Engine Soon” light will flash trouble codes.

NOTE: If only a code 12 is flashed, this means that all systems are working properly.

Trouble Code			Description
HFN01 Early	HFN01 Late HFN03 All	HFN02 All
12	12	12	No trouble. Diagnostic system is working properly (unless other codes follow).
13	13	13	Universal Exhaust Gas Oxygen (UEGO) Sense Cell Failure
14	14	14	Natural Gas Temperature (NGT) Input Voltage High
15	15	15	Natural Gas Temperature (NGT) Input Voltage Low
N/A	16	16	5 Volt Supply Air
17	17	N/A	Exhaust Back Pressure (EBP) Input Voltage High
18	18	N/A	Exhaust Back Pressure (EBP) Input Voltage Low
19	19	N/A	Exhaust Back Pressure (EBP) Higher than Expected
21	21	21	Foot Pedal Position 1 (FPP1) Input Voltage High
22	22	22	Foot Pedal Position 1 (FPP1) Input Voltage Low
23	23	23	Manifold Air Temperature (MAT) Input Voltage High
24	24	24	Manifold Air Temperature (MAT) Higher than Expected
25	25	25	Manifold Air Temperature (MAT) Input Voltage Low
26	26	26	Engine Coolant Temperature (ECT) Input Voltage High
27	27	27	Engine Coolant Temperature (ECT) Input Voltage Low
28	28	28	Engine Coolant Temperature (ECT) Higher than Expected
29	29	N/A	Exhaust Back Pressure (EBP) Lower than Expected
31	31	31	Foot Pedal Position 2 (FPP2) Input Voltage High
32	32	32	Foot Pedal Position 2 (FPP2) Input Voltage Low
33	33	33	Manifold Absolute Pressure (MAP) Input Voltage High
34	34	34	Manifold Absolute Pressure (MAP) Input Voltage Low
35	35	35	Barometric Pressure (BP) Higher than Expected
36	36	36	Barometric Pressure (BP) Lower than Expected
37	37	N/A	Preturbine Pressure (PTP) Input Voltage High
N/A	N/A	37	Upstream Throttle Pressure (UTP) Input Voltage High
38	38	38	Natural Gas Pressure (NGP) Input Voltage High
39	39	39	Natural Gas Pressure (NGP) Input Voltage Low
41	41	41	Natural Gas Pressure (NGP) Higher than Expected
42	42	42	Camshaft Position Sensor Error
43	43	43	Natural Gas Pressure (NGP) Lower than Expected
44	44	44	Closed Loop Multiplier Limit High
45	45	45	Closed Loop Multiplier Limit Low
46	46	46	Adaptive-Learn Multiplier Limit High
47	47	47	Adaptive-Learn Multiplier Limit Low
48	48	N/A	Preturbine Pressure (PTP) Input Voltage Low
N/A	N/A	48	Upstream Throttle Pressure (UTP) Input Voltage Low
49	49	49	Preturbine Pressure (PTP) Not Active
51	51	51	Random Access Memory (RAM) Error
52	52	52	Battery Voltage Low
53	53	53	Battery Voltage High
54	54	54	Throttle Position Sensor (TPS) Input Voltage High
55	55	55	Engine Control Unit (ECU) Error
56	56	56	Throttle Position Sensor (TPS) Input Voltage Low
57	57	57	Throttle Position Sensor (TPS) Higher than Expected
58	58	58	Universal Exhaust Gas Oxygen (UEGO) Heater Current High
59	59	59	Universal Exhaust Gas Oxygen (UEGO) Heater Current Low
N/A	N/A	61	Environmental Sensor Communications Failure
62	62	62	Universal Exhaust Gas Oxygen (UEGO) Pump Cell Voltage Low
63	63	63	Universal Exhaust Gas Oxygen (UEGO) Pump Cell Voltage High
64	64	N/A	Natural Gas Tank Temperature (NGTT) Voltage High
65	65	N/A	Natural Gas Tank Temperature (NGTT) Voltage Low
66	66	N/A	Natural Gas Tank Pressure (NGTP) Input Voltage High
67	67	N/A	Natural Gas Tank Pressure (NGTP) Input Voltage Low
68	68	68	Universal Exhaust Gas Oxygen (UEGO) Resistor Voltage High
69	69	69	Universal Exhaust Gas Oxygen (UEGO) Resistor Voltage Low
71	71	71	Boost Higher than Expected
72	72	72	Boost Lower than Expected
73	73	73	Overboost
74	74	74	Oil Pressure Switch Input Voltage Low
75	75	75	Injector Duty Cycle High
77	77	77	Natural Gas Temperature (NGT) Lower than Expected
78	78	78	Idle Validation Switch (IVS) Status OFF Unexpected
79	79	79	Idle Validation Switch (IVS) Status ON Unexpected
81	81	81	Throttle Position Sensor (TPS) Lower than Expected
83	83	N/A	Natural Gas Tank Pressure (NGTP) Higher than Expected
84	84	84	Vehicle Network Transmit Fault
85	85	85	Vehicle Network Receive Fault
86	86	86	Foot Pedal Position 1 (FPP1) Different than Expected
87	87	87	Foot Pedal Position 2 (FPP2) Different than Expected
N/A	94	94	Engine Control Unit (ECU) Internal Voltage Error
N/A	N/A	95	Knock Sensor Error
N/A	96	96	Natural Gas Pressure (NGP) Extremely High