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OBD0 Check Engine Light (CEL) Codes
When the Check Engine warning light has been reported on, turn the ignition on, pull down the passenger's side carpet from under the dashboard and observe the LED on the top of the ECU. The LED indicates a system failure code by blinking frequency. The ECU LED can indicate any number of simultaneous component problems by blinking seperate codes, one after another. Problem codes 1 through 9 are indicated by individual short blinks. Problem codes 10 through 44 are indicated by a series of long and short blinks. One long blink equals 10 short blinks. Add the long and short blinks together to determine the problem code.

you have to jump a service connector located on the passenger side right under the dashboard near the door. The connector has two pin inputs. Connecting them with something like a paper clip will force the check engine light on the dashboard to display the codes. when u pull the kick panel on the right side floor board where your ecu you there will be an extra plug hanging i beleive its green or blue, put the paper clip in it, turn the ignition to the on position and count the number of blinks on the check engine light.

codes 1-9 are indicated by a series of short flashes; two digit codes use a number of long flashes for the first digit followed by the number of short flashes for the second digit. So a code 43 will be represented by 4 long and 3 short flashes.

The position of codes in a sequence can be helpful in doing diagnostics. A display showing 1-1-1-pause-9-9-9 indicates two problems occurring at different times. A sequence showing 1-9-1-9-1-9 indicates two problems occurring at the same time.

Note: OBD0 is for ECUs in the years 1988 - 1991.

Indicator Blinks System Indicated
1, 2 Oxygen Content
3 Manifold Absolute Pressure
4 Crank Angle
6 Coolant
7 Throttle Angle
8 TDC Position
9 No. 1 Cylinder Position
10 Intake Air Temperature
13 Atmospheric Pressure
14 Electronic Air Control
15 Ignition Output Signal
17 Vehicle Speed Sensor
21 Spool Solenoid Valve
22 Oil Pressure Switch
43, 44 Fuel Supply System (KX, KS, KG)

If codes other than those listed above are indicated, count the number of blinks again. If the indicator is in fact blinking unusual codes, subsitute a known-good ECU and recheck. If the indication goes away, replace the original ECU. The Check Engine warning light and ECU LED may come on, indicating a system problem, when, in fact, there is a poor or intermittent electrical connection. First, check the electrical connections, clean or repair connections if necessary. If the Check Engine warning light is on and LED stays on, replace the ECU.
***this was taken from the Helms manual for 88-91 Civics.

Thanks to Hondata

OBD1 ECU Codes
Code Meaning
1 O2A - Oxygen sensor #1
2 O2B - Oxygen sensor #2
3 MAP - manifold absolute pressure sensor
4 CKP - crank position sensor
5 MAP - manifold absolute pressure sensor
6 ECT - water temperature sensor
7 TPS - throttle position sensor
8 TDC - top dead centre sensor
9 CYP - cylinder sensor
10 IAT - intake air temperature sensor
12 EGR - exhaust gas recirculation lift valve
13 BARO - atmospheric pressure sensor
14 IAC (EACV) - idle air control valve
15 Ignition output signal
16 Fuel injectors
17 VSS - speed sensor
19 Automatic transmission lockup control valve
20 Electrical load detector
21 VTEC spool solenoid valve
22 VTEC pressure valve
23 Knock sensor
30 Automatic transmission A signal
31 Automatic transmission B signal
41 Primary oxygen sensor heater
43 Fuel supply system
45 Fuel system too rich or lean
48 LAF - lean air fuel sensor
54 CKF - crank fluctuation sensor
58 TDC sensor #2
61 Primary oxygen sensor
63 Secondary oxygen sensor
65 Secondary oxygen sensor heater

How to convert an automatic ECU to 5 speed specs

First, let's find out if we have an automatic, or a 5 speed ECU. If you don't want to plug it in and throw code 19 (automatic trans lockup solenoid) to find out it is an automatic ECU, then look at the code on the side. If the second to last number is a 5, then you have an automatic ECU (for OBD-1 USDM ECUs). If the 2nd to last # is anything but a five you have an ECU for a 5 speed. If you have a jdm obd-1 ecu, look at the last 3 digits: 900 or 901 will be an automatic. If you were unfortunate enough to get an automatic ECU, then read on and learn how to cure all your problems.

For Jdm Pr3/Pw0 ECUs:
Move the jumper from R68 to R67 (both locations are to the left of the ROM)

For USDM OBD1 ECUs (p28):
Underneath the ROM there is a row of resistors labelled with RP__ . Remove RP17 and install a jumper across RP18.

RP18 (in the same location as usdm) needs to be lowered in value from 2.4k to 1.4k ohms.
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