How I Replaced The ECU and Ran Diagnostics

By Andy Woo


By Steve Chong

Created : 980818
Last updated: 990326

This page is in general how I get to the ECU either to replace it, or to use it to run diagnostic tests on the car.

It's actually fairly easy to get to. First and MOST importantly, I always disconnect the car's battery if I am planning on removing/disconnecting the ECU from the car. Paranoid as I am, I tend to disconnect both terminals of the battery, although it's not really necessary.

In case you didn't know, the battery is under the hood on the right side of the car (as seen from the driver's perspective). (It's on the left and side for RHD (right hand drive) cars).  The battery is on the same side as the passenger side (based on LHD versus RHD car).

Getting to the ECU

Now, I went to where the passenger places their feet and removed the top carpet (the one with the 'Z' sewn into it). This then exposes the car's main carpet which as you can see has two slits on either side of the passenger foot rest. So, NO cutting is necessary. The carpet is secured to a piece of wood underneath via three pieces of velcro. Merely pull the carpet back to expose the piece of wood.

passenger carpet peeled back

On the piece of wood, there are four screws (10 mm) holding it down. Remove these four screws and then remove the piece of wood to expose the area where the ECU lives.

ECU location

Removing the ECU

The ECU is held in place by a metal bracket. The bracket is secured by three screw/bolts (10 mm) . Remove these three screws. Once you remove the three screws, the bracket/ECU combination can be slowly and slightly moved out of its home. But just slightly, as the ECU is still connected by a wiring harness to the rest of the car.

You can remove this wiring harness/connector by loosening the screwbolt (10 mm) that is in the middle of the connector, this will allow you to remove the connector from its connection to the ECU.

wiring harness/connector attached to ECU

Once you have done this, you will notice that the ECU can now be easily removed from the ECU bracket by 'twisting' the ECU and the bracket in different directions. You do NOT have to bend any metal to do this.

separation of bracket and ECU

ECU Diagnostics

You do NOT have to remove the ECU, nor loosen the screws to its bracket, nor disconnect the ECU from the wiring harness/connector, nor disconnect the car's battery in order to run the ECU diagnostics. In fact, you have to make sure that the wiring harness/connector is connected and that the car's battery is connected as well!! :-).

Get a flat head screwdriver so that it can fit into the slotted potentiometer, which is located on the right hand side of the ECU. The oblong hold just above the potentiometer houses an LED that will flash codes at you. This flashing is mirrored by the 'engine' light on the dash.

ECU Diagnostic Control & Status LED

To enter the diagnostic mode, turn the ignition switch to the 'on' position, but do not start the car. Confirm that the LED is lit.  The ECU is now in what is called "mode I".

You are now in "mode II".

Read the resulting codes as it is being flashed via the LED, or the dash 'engine' light. It will flash the first digit with long flashes and then the second digit with short flashes. For example 4 long flashes followed by 3 short flashes means the number 43.

Note that more than one coding can be displayed, i.e. 11 may be displayed, followed by 21, in which case check the crank angle sensor first.

Here is a table of the codes with their meanings :

11: Crankshaft position sensor circuit
12: Mass air flow sensor circuit
13: Engine coolant temperature sensor circuit
14: Vehicle speed sensor circuit
21: Ignition signal circuit
*32: EGR function
33: Heated oxygen sensor circuit (Left side)
34: Knock sensor circuit
*35: EGR temperature sensor circuit
42: Fuel temperature sensor circuit
43: Throttle position sensor circuit
*45: Injector leak
*51: Injector circuit
53: Heated osygen sensor circuit (Right side)
54: Signal circuit from A/T control unit to ECM/ECU (A/T only)
55: No malfunctions in the above circuits

* : Not available for non-california models.

A little more meaning :

11 - crank angle sensor circuit, 1 deg or 120 deg signal is not received for the first few seconds during engine cranking or not often enough at high RPM- check harness, connectors and the crank angle sensor.
12 - Air flow meter circuit, open or short circuit is detected- check harness, connectors and the air flow meter.
13- engine temperature circuit, open or short circuit is detected- check harness, connectors and sensor.
21 - ignition signal circuit, the ECU is not detecting ignition signal from the transistor ignition unit- check harness, connectors and power transistor unit.
33- Oxygen sensor, - check harness, connectors and sensor.
34 - detonation sensor circuit , open or short circuit is detected- check harness, connectors and sensor.
42 - Fuel temperature sensor circuit, open or short circuit is detected- check harness, connectors and sensor.
43 - throttle sensor circuit, open or short circuit is detected- check harness, connectors and sensor.
54 - signal circuit from the auto transmission computer is open or short circuit check harness & connector. (Auto only)

When a particular fault is detected, it's held in memory for 50 engine starts, then deleted. If the fault disappears, then reappears, the count to 50 starts again. If at this point you want to retain the diagnostic result in memory, turn the ignition off, and wait 5 seconds for the ECU power supply relay to drop out. The ECU will drop out of mode II, and next time its turned on, it will start up in mode I. If you disconnect the battery for more than 24 hours, the fault codes will be lost in the ECU's back up memory.

The next thing to check is the exhaust gas sensors. When the LED is on, the fuel mixture condition in the exhaust gas is lean, when its off, its rich. At this point the LED is displaying the results for the left side sensor. If all is OK, the LED will flash at least 5 times in every 10 seconds. If you're all done, you can turn the engine & ignition off from here, or go back to any of the steps above. You can't however go from mode I to II or back again while the engine is running. To change modes, switch the engine off, but not the ignition, first.

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