To ensure correct diagnosis and repair, testing should be done in the following order:

Get information about conditions present when malfunction occurred. Retrieve and record DTCs and freeze-frame data. See RETRIEVING DIAGNOSTIC TROUBLE CODES . If MIL is on, and DTCs are not accessible, check for ECM fail-safe mode. See FAIL-SAFE MODES . If symptoms exist, but DTCs or freeze-frame data do not exist, go to next step. If symptoms do not exist, but DTCs or freeze-frame data exist, go to step 5 .
Attempt to duplicate conditions present when malfunction occurred. If malfunction is detected, perform basic inspection. See BASIC INSPECTION under THROTTLE POSITION SENSOR, IGNITION TIMING & IDLE SPEED in ON-VEHICLE ADJUSTMENTS article. If using CONSULT-II, go to next step. If using generic scan tool, go to step 4 .
Check inputs and outputs in DATA MONITOR (SPEC) mode with CONSULT-II. See DATA MONITOR (SPEC) MODE under CONSULT-II under SCAN TOOL USAGE. Compare readings with specifications listed in PIN VOLTAGE/PID VALUE CHARTS article. If problem is found, repair as necessary. If problem is not found, go to next step.
Go to TROUBLE SHOOTING - NO CODES article for diagnosis by symptom. If malfunction cannot be verified, problem may be intermittent. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES article.
Clear DTCs and freeze-frame data. See CLEARING DIAGNOSTIC TROUBLE CODES . Attempt to reset DTC by performing DTC CONFIRMATION TEST in appropriate diagnostic test. If multiple DTCs are present, diagnose in order of priority. See MULTIPLE DTC PRIORITY IDENTIFICATION table. If DTC does not reset, problem may be intermittent. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES article. If DTC resets, go to next step.
Perform appropriate diagnostic procedure. See DIAGNOSTIC TESTS . After repairs are made, perform appropriate DTC CONFIRMATION TEST to verify repair. Clear DTCs. If SRT codes are not set, drive vehicle in specified driving pattern to set all SRT codes. See SYSTEM READINESS TEST .

MULTIPLE DTC PRIORITY IDENTIFICATION

Priority	DTC
1	U1000, U1001, P0101, P0102, P0103, P1102, P0112, P0113, P0127, P0117, P0118, P0125, P0128, P0122, P0123, P0222, P0223, P1225, P1226, P1229, P2135, P2122, P2123, P2127, P2128, P2138, P0181, P0182, P0183, P0327, P0328, P0332, P0333, P0335, P0340, P0460, P0500, P0705, P1610-P1615, & P1706
2	P0031, P0032, P0051, P0052, P0037, P0038, P0057, P0058, P0132-P0134, P0152-P0154, P1143, P1144, P1163, P1164, P0138, P0139, P0158, P0159, P1146, P1147, P1166, P1167, P0441, P0444, P0445, P1444, P0447, P1446, P0451-P0453, P0550, P0710, P0720, P0740, P0744, P0745, P1716, P1720, P1730, P1752, P1754, P1757, P1759, P1762, P1764, P1767, P1769, P1722, P1744, P1065, P1111, P1136, P1122, P1124, P1126, P1128, P1140, P1145, P1217 & P1805
3	P0011, P0021, P0171, P0172, P0174, P0175, P0300-P0308, P0420, P0430, P0442, P0456, P0455, P0506, P0507, P1121, P1135, P1148, P1168, P1211, P1212, P1564 & P1568, P1572, P1574 & P1780

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Testing Procedure

Use the Manual With the Right Hardware

When to See a Mechanic

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