Check Engine Light Diagnosis

Diagnose the cause of an illuminated Check Engine Light on a 1992 Dodge Viper by retrieving diagnostic trouble codes and performing basic troubleshooting to identify the fault.

Warnings

Tools required

Parts

• Replacement sensor (if diagnosed as faulty) × 1 — Varies by fault code

Preparation

1. Ensure vehicle is parked on level ground in a well-ventilated area
2. Allow engine to cool if recently driven to prevent burns during inspection
3. Gather service manual or wiring diagrams specific to 1992 Viper for code interpretation
4. Have paper and pen ready to record trouble codes and freeze frame data

Procedure

1. 1

   Verify Check Engine Light Condition
   Turn ignition key to the ON position without starting the engine. Observe the Check Engine Light (Malfunction Indicator Lamp) on the instrument cluster. The light should illuminate briefly during bulb check, then extinguish if no codes are stored. If the light remains illuminated or flashes, codes are present. Note the pattern if flashing occurs.
2. 2

   Access Diagnostic Connector
   Locate the diagnostic connector under the dashboard on the driver's side. The 1992 Viper uses a Chrysler-style diagnostic connector; note that on many early-90s Chrysler products this connector is located in the engine compartment rather than under the dash. Verify exact connector location and pinout against the service manual. Identify the two pins required for code retrieval: the diagnostic request pin and the ground pin. Refer to service manual for exact pin locations on this connector.
3. 3

   Retrieve Diagnostic Trouble Codes
   Chrysler vehicles of this era use the key-cycle method: turn the ignition ON-OFF-ON-OFF-ON within approximately 5 seconds, leaving it in the final ON position (do not start engine). Alternatively, connect a Chrysler DRB-II scan tool to the diagnostic connector. Watch the Check Engine Light on the instrument panel. The light will flash two-digit codes: a series of flashes for the first digit, a pause, then flashes for the second digit, followed by a longer pause before the next code. Record all codes displayed. Code 55 indicates end of code sequence. Codes will repeat continuously. Turn ignition to ON position (do not start engine). Watch the Check Engine Light on the instrument panel. The light will flash two-digit codes: a series of flashes for the first digit, a pause, then flashes for the second digit, followed by a longer pause before the next code. Record all codes displayed. Code 55 or a steady pattern indicates end of code sequence. Codes will repeat continuously.
4. 4

   Interpret Code Definitions
   Cross-reference retrieved codes with the 1992 Viper diagnostic code chart. Common codes include: Code 12 (battery disconnect within last 50 starts), Code 21 (oxygen sensor signal), Code 22 (coolant temperature sensor), Code 24 (throttle position sensor), Code 25 (idle air control / automatic idle speed motor circuit — verify definition against Viper-specific code chart), Code 27 (injector control circuit), Code 33 (air conditioning clutch relay), Code 37 (transmission-related — not applicable to the manual-transmission Viper; disregard if present), Code 42 (fuel pump relay), Code 43 (ignition coil control), Code 44 (battery temperature sensor), Code 51 (lean exhaust condition), Code 52 (rich exhaust condition), Code 53 (internal PCM failure), Code 55 (end of message). Multiple codes may be stored.
5. 5

   Perform Visual Inspection
   With codes identified, perform a thorough visual inspection of the affected circuit or sensor. Open the hood and inspect wiring harnesses for damage, abrasion, or rodent activity. Check all vacuum lines for cracks, disconnections, or collapse. Inspect electrical connectors for corrosion, bent pins, or loose connections. Pay special attention to the air intake system for leaks between the throttle body and intake manifold gaskets. Look for obvious damage to sensors indicated by trouble codes.
6. 6

   Test Sensor Circuits with Multimeter
   Based on retrieved codes, test the specific sensor or circuit indicated. With ignition OFF, disconnect the suspect sensor connector. Use a digital multimeter to check sensor resistance values and compare against factory specifications. For example, measure coolant temperature sensor resistance and compare against the factory resistance-vs-temperature chart in the Viper service manual. Check wiring continuity from sensor connector to PCM by measuring resistance on the signal wire (should be less than 5 ohms). Check for short to ground by measuring resistance from signal wire to chassis ground (should be infinite/OL).
7. 7

   Inspect Oxygen Sensor Operation
   If Code 21, 51, or 52 is present, inspect the oxygen sensor located in the exhaust system. With engine at operating temperature and running, use a multimeter to measure oxygen sensor voltage at the sensor connector (backprobe the signal wire without disconnecting). Voltage should fluctuate between approximately 0.1 and 0.9 volts as the engine runs in closed-loop mode. A steady voltage indicates a failed sensor or an actual rich/lean condition. Inspect exhaust system for leaks upstream of the oxygen sensor that could affect readings.
8. 8

   Check Throttle Position Sensor
   If Code 24 is present, test the throttle position sensor (TPS) located on the throttle body. With ignition ON and engine OFF, backprobe the TPS signal wire with a multimeter set to DC voltage. At closed throttle, voltage should read approximately 0.5-1.0 volts. Slowly open the throttle to wide-open position while observing voltage; it should smoothly increase to approximately 4.0-4.5 volts. Any sudden jumps, drops, or dead spots indicate a faulty TPS requiring replacement.
9. 9

   Test Coolant Temperature Sensor
   If Code 22 is present, test the coolant temperature sensor located on the engine block or thermostat housing. With engine cold and ignition OFF, disconnect the sensor and measure resistance across the sensor terminals. Compare resistance to the resistance-vs-temperature chart in the Viper service manual for the measured coolant temperature. If resistance is out of specification or reads open/shorted, replace the sensor. If sensor tests good, check wiring from sensor to PCM for opens or shorts.
10. 10

    Inspect Fuel and Ignition Systems
    If codes indicate ignition coil control (Code 43) or injector control (Code 27), perform basic fuel and ignition system checks. Verify fuel pressure at the fuel rail using a fuel pressure gauge, comparing the reading to the 1992 Viper factory fuel pressure specification (confirm exact value in the service manual before proceeding). Check ignition coil primary and secondary resistance with multimeter. Inspect the ignition coil(s), spark plug wires, and spark plugs for wear, carbon tracking, or damage. Note that the 8.0L V10 does not use a conventional distributor cap and rotor; verify ignition system configuration against the service manual. Test injector resistance (typically 12-16 ohms) and listen for injector clicking with engine running.
11. 11

    Clear Diagnostic Codes
    After completing repairs or if codes are determined to be historical/intermittent, clear stored codes from PCM memory. Remove the jumper wire or disconnect the code reader. Disconnect the negative battery cable and wait 30 seconds to allow capacitors to discharge and clear PCM memory. Reconnect the battery. Note that disconnecting the battery will also set a Code 12 (battery disconnect) on the next key-on, which is normal.
12. 12

    Verify Repair and Road Test
    Start the engine and verify the Check Engine Light illuminates during bulb check, then extinguishes after 2-3 seconds. Allow engine to idle for at least 5 minutes while monitoring for light re-illumination. If light remains off, perform a road test under various driving conditions (idle, acceleration, deceleration, highway speed) for at least 10-15 minutes. After road test, retrieve codes again using the same procedure as Step 3. If Code 55 or no codes appear, repair is successful. If original codes return, further diagnosis is required. Note: All tested engine sensors should read within normal operating parameters when checked with a multimeter or the Chrysler DRB tool

Reassembly

1. Reconnect any sensors or connectors that were disconnected during testing
2. Secure all wiring harnesses with original clips or ties to prevent chafing
3. Ensure all vacuum lines are properly routed and connected
4. Replace any damaged sensors or components identified during diagnosis
5. If battery was disconnected, reset clock and radio presets

Verification

• Check Engine Light should illuminate during key-ON bulb check, then extinguish within 2-3 seconds
• No diagnostic trouble codes should be present when retrieved after repair
• Engine should idle smoothly with no hesitation or surging
• All engine sensors should read within normal operating parameters on scan tool or multimeter
• Complete a drive cycle including idle, acceleration, and highway driving with no Check Engine Light illumination

Related trouble codes on this vehicle

Codes that commonly send this job to the bay — tap one for symptoms, causes, and diagnostic steps.

Chasing one of these codes and not sure which part is guilty? Ask a master mechanic about YOUR car →

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