Turn your car off. Look under your dash in the passenger footwell, and you'll find a single plug in a green boot. You don't even have to pull back the carpet to find it. Jump the two terminals with a paperclip or some other metal thing. Flip on the ignition, but not the engine. The CEL (check engine light) will start flashing, morse code style (dash/dot). Count the number of long flashes and short flashes. The code will repeat. In my case I got 4 long flashes and 1 short flash. Hence code 41.
A website with all codes for honda
http://www.geocities.com/honda_club_egypt/ecu.htm
Code Accord Civic CRX Prelude Error
1 85-98 86-98 85-91 85-98 Oxygen Sensor "A" (Primary)
defective circuit or unplugged / defective sensor
2 85-89 86-87 85-87 NA Oxygen Sensor "B"
defective circuit or unplugged / defective sensor
3 85-98 86-98 85-91 85-98 MAP Sensor (Manifold Absolute Pressure)
defective circuit or unplugged / defective sensor
4 90-98 88-98 88-91 85-98 CKP Sensor (Crankshaft Position Sensor)
defective circuit or unplugged / defective sensor
5 85-98* 86-98 85-91 85-98 MAP Sensor (Manifold Absolute Pressure)
mechanical problem / disconnected piping
* exc 94-95
6 85-98 86-98 85-91 85-98 ECT Sensor (Engine Coolant Temperature)
defective circuit or unplugged / defective sensor
7 85-98 86-98 85-91 85-98 TP Sensor (Throttle Position)
defective circuit or unplugged / defective sensor
8 85-98 86-98 85-91 85-98 TDC Sensor (Top Dead Center)
defective circuit or unplugged / defective sensor
9* 85-98** 86-98 85-91 85-98 CYP Sensor (Cylinder)
defective circuit or unplugged / defective sensor
* not a valid code for any throttle body injection Hondas
** not a valid code for V-6 engines
10 85-98 86-98 85-91 85-98 IAT Sensor (Intake Air Temperature)
defective circuit or unplugged / defective sensor
12 85-98 86-98* 85-91* 85-98 EGR Lift Sensor (Exhaust Gas Recirculation)
defective circuit or unplugged / defective sensor
* not a valid code if car is not equipped with an EGR valve (some Civic models are not)
13 85-98 86-98 85-91 85-98 BARO Sensor (Atmospheric Pressure)
defective circuit or unplugged / defective sensor
14 88-98 88-98 88-91 88-98 IAC Valve (Idle Air Control)
defective circuit or unplugged / defective sensor
15 90-95 88-95 88-91 88-95 Ignition Output Signal
missing or defective ignition output signal
16 90-95 88-95 88-91 88-95 Fuel Injector System
defective circuit or unplugged / defective fuel injector
17 88-98 88-98 88-91 88-98 VSS (Vehicle Speed Sensor)
defective circuit or unplugged / defective sensor
19 NA 88-95 88-91 NA Automatic Transmission Lock Up Control Solenoid Valve
defective circuit or unplugged / defective solenoid valve
20 90-98 88-98 88-91 92-98 Electrical Load Detector
defective circuit or unplugged / defective sensor
21* 94-98** 92-95 NA 93-95 VTEC Solenoid Valve
defective circuit or unplugged / defective solenoid valve
* not a valid code if car does not have a VTEC engine
** not with V-6 engine
22* 94-98 92-98 NA 92-98 VTEC Oil Pressure Switch
defective circuit or unplugged / defective oil pressure switch
* not a valid code if car does not have a VTEC engine
23 98* 96-98 NA 92-98 KS (Knock Sensor)
defective circuit or unplugged / defective sensor
* not a valid code if V-6 engine
30 90-98* NA NA 92-98* Automatic Transmission Signal: "A" / SEAF / SEFA / TMA or TMB
defective circuit or unplugged / defective sensor
* not a valid code for 1996
31 90-95 NA NA 92-95 Automatic Transmission Signal "B"
defective circuit or unplugged / defective sensor
41 90-98 92-98 NA 90-98 Primary Oxygen Sensor - Heater
circuit malfunction
43 90-95 92-95 NA 90-95 Fuel Supply System
defective or malfunctioning fuel supply system
45 96-98 96-98 NA 96-98 System Too Lean or Too Rich
malfunction in the fuel monitoring systems
48 98 92-98 NA NA LAF Sensor (Lean Air Fuel)
defective circuit or unplugged / defective sensor
54 96 96-98 NA NA CKF Sensor (Crankshaft Speed Fluctuation)
defective circuit or unplugged / defective sensor
58 98* NA NA NA TDC Sensor 2 (Top Dead Center)
defective circuit or unplugged / defective sensor
* with V-6 engine only
61 96-98 96-98 NA 96-98 Oxygen Sensor, Heated - Sensor 1 (Primary)
high voltage, low voltage, or slow response
63 96-98 96-98 NA 96-98 Oxygen Sensor, Heated - Sensor 2 (Secondary)
high voltage, low voltage, or slow response
65 96-98 96-98 NA 96-98 Oxygen Sensor Heater (Secondary)
malfunctioning or defective oxygen sensor heater
67 96-98 96-98 NA 96-98 Catalyst system Efficiency Below Threshold
malfunctioning or defective catalyst system
70 96-98 96-98 NA 96-98 Automatic Transmission
malfunction with the automatic transmission controls
71 96-98 96-98 NA 96-98 Cylinder 1 Misfire or a Random Misfire
a condition is present that is creating a cylinder misfire
72 96-98 96-98 NA 96-98 Cylinder 2 Misfire or a Random Misfire
a condition is present that is creating a cylinder misfire
73 96-98 96-98 NA 96-98 Cylinder 3 Misfire or a Random Misfire
a condition is present that is creating a cylinder misfire
74 96-98 96-98 NA 96-98 Cylinder 4 Misfire or a Random Misfire
a condition is present that is creating a cylinder misfire
75 98* NA NA NA Cylinder 5 Misfire or a Random Misfire
a condition is present that is creating a cylinder misfire
* valid code for V-6 engine only
76 98* NA NA NA Cylinder 6 Misfire or a Random Misfire
a condition is present that is creating a cylinder misfire
* valid code for V-6 engine only
80 96-98 96-98 NA 96-98 Exhaust Gas Recirculation
insufficient flow detected
86 96-98 96-98 NA 96-98 ECT Sensor (Engine Coolant Temperature)
circuit range / performance problem
90 97-98 96-98 NA 97-98 Evaporative Emission Control System
leak detected in the fuel tank area
91 96-98 96-98 NA 97-98 Fuel Tank Pressure Sensor
low input
92 96-98 96-98 NA 96-98 Evaporative Emission Control System
insufficient purge flow