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Checking ECU Error Codes


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#1 87cquest

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Posted 28 November 2002 - 12:56 PM

This question gets asked a lot by people, and I was hoping we could put it in the FAQ section.

The easiest and cheapest way to check for ECU error codes is to go to Radio Shack and buy a 12v LED for about $2.50.  
http://www.cgwebdesi...m/quest/led.jpg

Once you have your LED, it's best to solder aligator clips onto each wire.  I've heard you can go without them, but I've never got it to work without them.  

Once you have your LED ready to roll, it's time to test for codes.  I was surprised as to how easy it was to check for these.  

Open the glove box, and squeeze the sides to drop it all the way down.  Get down and look up and you will see a white/yellowish colored plug with nothing plugged into it.  Take the LED and hook the red lead up to the most upper right terminal, and the black lead to the most lower left terminal.  Once you have done this, you're just about done.  Just start the car, or turn the key to the "On" position.  

If there are any errors, the LED will blink a certain amount of times.  The amount of blinks determines the error.  For example, 1 blink then a pause, then one blink again, is code #1.  3 blinks, then a pause, then 3 blinks, again, and so on and so forth, is code #3.  Note - It will always give you a code 1 (o2 sensor) until the car warms up, and the car must be started to test this.

Here are the codes:
1 - o2 Sensor
2 - Ignition pulse
3 - MAS
4 - n/a
5 - TPS
6 - ISC
7 - Engine coolant temp sensor (two prongs, not one)

After you've checked the codes, and fixed whatever was wrong, unplug the battery for a minute to reset your ECU.  Hope this helps.

~Carlos





#2 zactek

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Posted 29 November 2002 - 06:35 PM

I don't know how the plug looks on other people's car, but on mine it faces straight down towards the floor. In this case, would the "upper" row of pins be considered the ones  towards the back of the car? Thanks.
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#3 87cquest

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Posted 29 November 2002 - 11:07 PM

Well, I usually lay down on the seat (on my back), and look straight up at it.  So the upper pins are the ones farthest from you when sitting normally in the seat.  Hope this helps.

~Carlos

#4 zactek

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Posted 30 November 2002 - 06:06 PM

That's exactly the answer I was looking for, thanks, Carlos. I just bought the LED today and soldered a miniature alligator clip (also from Radio Shack)onto each lead. Should make it much easier to check codes now. Thanks!
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#5 87cquest

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Posted 01 December 2002 - 10:21 AM

No problem.  You could even drive with it now with aligator clips.  Good luck with that ~_^

#6 blue_crush

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Posted 09 February 2003 - 01:45 PM

mine just blinks and blinks and blinks, no pauses, so does that mean the my O2 sesor is bad, or does it mean that im doin something wrong?
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#7 Edde

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Posted 10 February 2003 - 02:00 PM

If it blinks steady and does not go solid "on" after just a minute or two (running) that would be an 02 code, yes...  if you are having driveability issues then this would be a place to start.

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#8 StarWoes

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Posted 02 March 2003 - 03:37 PM

are all years of this car checked this way?  i think i saw somehere that earlier models have the connectors in the engine bay....not sure but someone who knows breakdown for us which years have the connector behind the glove box and which years don't.

#9 Edde

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Posted 04 March 2003 - 12:01 AM

all 87-89 are the same at least.  I think the older ones are under the hood...
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#10 MikeMeyerhoff

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Posted 14 March 2003 - 11:49 PM

A really slick way to hardwire this would be to get an "illuminated" type of switch from radioshack or someplace.   Make sure to get a 12v one, if its 120 v the light may be too dim...

That type of switch has 3 connections:  a "12 volt in",  a "12 volt switched out", and a "to ground"

Simply wire one of the wires mentioned by 87cquest to the "12 volt in" and the other to the "to ground" and you've got a switch that gives you error codes smile.gif

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#11 zactek

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Posted 18 March 2003 - 04:22 PM

I went to check my codes today. My diag. connector only has 5 pins, is this right??? Anyway, I plugged in the LED and waited for about 10 minutes with the engine running. Nothing lit or blinked at any point. Is there something wrong OR is there nothing wrong at all? Doesn't the LED supposed to light at first when you turn the key to ON?? Thanks, my ISC has been acting up so I decided to check it.
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#12 Edde

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Posted 19 March 2003 - 10:18 AM

if it's the in dash connector, it should work as long as you are using a 12V DC LED,
not a high voltage AC unit.

I just had a chance to look, 5 pins it is.  You can also check cruise and ETACS with this connector.
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#13 zactek

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Posted 19 March 2003 - 08:39 PM

I did it again today, I reversed the wires the other time 'cause I was upside down, hehe. After the car was warmed up, the 12V LED kept blinking, with a 2 sec interval. Blink, pause , blink, pause. It looked like 1 blink, no other intervals. Does this mean it's the O2 code? I have a 3-wire sensor which is fairly new,and the car doesn't run rich at all, except when on boost and a little when idling (I'm using a Halmeter)
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#14 Edde

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Posted 20 March 2003 - 08:45 AM

Under normal conditions it should go steady ON when warmed up, and a single flash repeated means 02.  I can't say tho how a three wire might or might not affect the readings (?)

Have you pulled the neg battery cable after doing any work - like changing sensors?

You would need to in order to clear the memory of that code...
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#15 zactek

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Posted 20 March 2003 - 08:27 PM

Thanks alot, yeah, I always pull the neg. cable after resets or changing sensors. The LED had a solid light on at first, then blinked once. I don't know if the 3-wire acts different, either, but that's the least of my problems, the car doesn't run overly rich so I'm not too worried. As long as there isn't other codes that come up.
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#16 dfowler21

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Posted 28 April 2003 - 02:51 PM

Anyone know where the diagnostic connector is on an 84?  I cannot find mine to save my life.  Ive dropped the glove box and its not there.  I couldnt find anything under the hood.   I also didnt see it in the drivers side footwell where my DSMs plug is located.  Anyone got any ideas???

#17 Edde

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Posted 28 April 2003 - 06:48 PM

some earlier models have one under the hood but I can't help ya.  I would ask
on the VM or BS if you haven't 'cause this might not get seen by many.
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#18 shift1313

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Posted 14 July 2003 - 04:13 PM

not sure if anyone is still watching this but my 84 has a connector under the steering column above the pedals.  havent checked the codes but seems like thats what it is.
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#19 starquestPilot

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Posted 26 July 2003 - 05:39 AM

Hmmm - under the steering column?  I dropped my column to replace a bad ignition switch; didn't happen to notice a "stray" connector - then again, I wasn't looking for one...

However, I have looked for the diagnostics connector on my '84 - couldn't find it.  So, I pulled the kick panel cover off at the ECU and just stuck the VOM probes right in the connector.  It would be nice to have hard-wired indicator..

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#20 indy_85stariones

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Posted 26 July 2003 - 10:47 AM

For 85/86 model years, the ECU diagnostic connector is a round connector located near the air filter canister -- right next to the fuel pump test connector.
For 83 model year, there is no diagnostic connector.  I couldn't even find an ECU  diagnosis wire in the 83 wiring diagram.  
For 84 model year, you may have to tap at the ECU connector as posted earlier.
----------------------------------------------------------------------------------------------------
Page 14-23 of my 85 Conquest Manual gives instructions for using voltmeter and
Page 14-49 of Fuel (ECI) System section of my 88 Conquest Manual

1) Turn ignition switch to OFF.

2A) - for 86 and older
...1. Position self-diagnosis 3 pin connector such that,
the single pin is at the top and the dual pins are at the bottom. Connector is
located in engine compartment near Air Can and Fuel Pump diagnosis 2 pin connector.
...2. Connect voltmeter positive to upper single pin (signal, light blue+yellow wire). Connect negative to lower right pin (ground, black wire). FYI - lower left pin (white wire) is O2 sensor diagnosis connector.

2B) - for 87 and newer
...1. Locate the self-diagnosis 12 pin connector behind glove box.
...2. Connect voltmeter positive to upper right corner pin (signal, black+red wire).
Connect negative to lower left corner pin (ground, black wire).

3) Turn ignition switch to ON. The contents of memory will immediately start. Under normal error-free conditions, the pointer of voltmeter constantly indicates 12V. Abnormal conditions, count the number of 12V pulses between 2 second 0V readings. For example -- Two 12V pulses + 2 sec 0V + 4 12V pulses + 2 sec 0V = error codes 2 and 4 = bad ignition and bad Pressure Sensor.

4) Turn ignition switch to OFF.

5) After replacing/repairing defective part, disconnect negative terminal of battery for 15 seconds to erase abnormal error code.


01 - O2 sensor
02 - Ignition Pulse (engine speed sensor)
03 - Air Flow sensor
04 - Boost Pressure sensor (only for 86 and older)
05 - Throttle Position sensor
06 - ISC motor position sensor
07 - Intake Coolant Temp sensor







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