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Checking the 'Check Engine' Light (5th GEN Prelude)

Product Review

By Mark Higgins (aka DirtyLude)
Last Updated


When the computer (ECU) in your car detects a problem with how the car is running it reports it by lighting up the yellow engine light on your dash. Any Joe Schmoe can actually check what the ECU is complaining about as long as they know how.

Instructions and pictures to find the 3rd and 4th Gen service connectors will be coming soon.


In order to get the code you'll need to find the service connector. On the 5th Gens it's on the right hand side of the centre console underneath a carpeted panel that just pops right out. The connector looks like a telephone connector and is plugged into a blue plastic piece that just serves as a holder. Unplug it and you'll see that it has two sockets in it that lead to two wires.

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You'll need a small piece of wire for this. Make sure the car is off. Just connect the two ports in the service connector together with the wire. After you've shorted the service connector, turn the key in the ignition to the on position (you don't actually need to start the car) and the yellow engine light will flash at you. There will be a series of long flashes, short flashes, and then it will repeat itself. Count the number of long flashes and short flashes and this will give you the MIL code. 2 long flashes and 3 short flashes is code 23. Once you've found the code, turn the key back to off and remove the wire from the service connector. You can look up the code in the following table.


Code Detection Item Probable Cause
3 Manifold Absolute Pressure Signal
  • High/Low Signal
  • Open or short in MAP sensor circuit
  • MAP sensor
  • ECM
10 Intake Air Temperature Cicuit
  • Range/Performance Problem
  • Open or short in IAT sensor
  • IAT sensor
  • ECM
86 Engine Coolant Temperature Circuit
  • Range/Performance Problem
  • ECT sensor
  • Cooling System
6 Engine Coolant Temperature Circuit
  • High/Low Signal
  • Open or short in ECT sensor
  • ECT sensor
  • ECM
7 Throttle Position Circuit
  • High/Low Signal
  • Open or short in TP sensor
  • TP sensor
  • ECM
1 Primary Heated Oxygen Circuit
  • High/Low Signal
  • Open or short in Primary HO2S sensor
  • Primary HO2S sensor
  • ECM
61 Primary Heated Oxygen Circuit
  • Slow response
  • Primary HO2S sensor
  • Exhaust System
41 Primary Heated Oxygen Circuit
  • Circuit Malfunction
  • Open or short in Primary HO2S sensor heater circuit
  • ECM
63 Secondary Heated Oxygen Circuit
  • High/Low Signal
  • Slow response
  • Open or short in Secondary HO2S sensor
  • Secondary HO2S sensor
  • Exhaust System
  • ECM
65 Secondary Heated Oxygen Circuit
  • Circuit Malfunction
  • Open or short in Primary HO2S sensor heater circuit
  • ECM
45 System too Rich/Lean
  • Fuel Supply System
  • Primary HO2S sensor
  • Contaminated Fuel
  • Valve Clearance
  • Exhaust Leakage
71,72,73,74 Misfire Detected -
  • Cylinder 1
  • Cylinder 2
  • Cylinder 3
  • Cylinder 4
  • Ignition System
  • Fuel Supply System
  • EGR System
  • IAC Valve
  • Contaminated Fuel
  • Lack of Fuel
  • Valve Clearance
  • Fuel Injector Circuit
  • Low Compression
23 Knock Sensor Circuit Malfunction
  • Open or short in Knock Sensor (KS) circuit
  • Knock Sensor (KS)
  • ECM
4 Crankshaft Position Sensor Circuit
  • Malfunction / Range / Performance
  • Crankshaft Position Sensor
  • Crankshaft Position Sensor circuit
  • ECM
  • Timing Belt skipped teeth
80 Exhaust Gas Recirculation
  • Insifficient flow detected
  • EGR Valve
  • EGR Line
67 Catalyst System Efficiency Below Threshold
  • Three way Catalytic Converter
  • Secondary HO2S sensor
92 Evaporative Emission Control System
  • Insifficient Purge Flow
  • Purge Flow Switch Malfunction
  • EVAP Purge Control Solenoid Valve
  • EVAP Purge Control Solenoid Valve Circuit
  • EVAP Purge Flow Switch
  • EVAP Purge Flow Switch Circuit
  • Throttle Body (Purge Port)
  • Tubing
  • Vacuum connections
  • ECM
91 Fuel Tank Pressure Sensor High/Low Input
  • Open or short in Fuel Tank Pressure Sensor Circuit
  • Fuel Tank Pressure Sensor
  • ECM
17 Vehicle Speed Circuit Malfunction
  • Vehicle Speed Sensor
  • Vehicle Speed Sensor Circuit
14 Idle Control System Malfunction
  • IAC Valve
  • Fast Idle Thermo Valve
  • Throttle Body
70 Automatic Transaxle
  • Automatic Transmission
13 Barometric Pressure Circuit
  • Range/Performance Problem
  • ECM (Baro Sensor)
5 Manifold Absolute Pressure
  • Lower/Higher than Expected
  • MAP Sensor
22 VTEC System Malfunction
  • VTEC Solenoid Valve
  • Open or Short in VTEC Solenoid Valve circuit
  • VTEC Pressure Switch
  • Open or Short in VTEC Pressure Switch Circuit
  • ECM
20 Electrical Load Detector Circuit
  • Low/High Input
  • Electrical Load Detector
  • Electrical Load Detector Circuit
  • ECM
8 Crankshaft Position / Top Dead Center Sensor
  • Connector Disconnection
  • Intermittent Interruption
  • Sensor No Signal
  • CKP/TDP Sensor
  • TDC Sensor Circuit
  • ECM
9 Cylinder Position Sensor
  • Intermittent Interruption
  • No Signal
  • CYP Sensor
  • CYP Sensor Circuit
  • ECM
90 Evaporative Emission Control System
  • Leak Detected (Fuel Tank System)
  • Leak Detected (EVAP Control Canister System)
  • Fuel Fill Cap
  • Vacuum connection
  • Fuel tank
  • Fuel tank pressure sensor
  • EVAP bypass solenoid valve
  • EVAP two way valve
  • EVAP control canister ventshut valve
  • EVAP control canister
  • EVAP purge control solenoid valve
92 Evaporative Emmision Purge Flow Switch
  • Malfunction
  • EVAP Purge flow switch
  • EVAP Purge flow switch circuit
  • Vacuum connections
  • ECM
12 EGR Valve Lift Sensor
  • Lift Insifficient Detected
  • High Voltage
  • EGR Valve (with lift sensor)
  • EGR valve lift sensor circuit
  • EGR control solenoid valve
  • EGR control solenoid valve circuit
  • EGR line
  • ECM
14 Idle Air Control Valve Circuit Failure
  • IAC Valve
  • IAC Valve circuit
  • ECM
30 SEAF/SEFA Signal Line Failure Open or short in SEAF/SEFA circuit