Hi All,

First off excuse me if this question has been already been covered.

We have a 93 245 with the check engine light on. It has been on at least 2 years.

The code its showing is 1-2-3. If I disconnect the battery the light returns on after 90 seconds or so.

Being in California we need the check engine light out to pass smog. Our car has always passed smog.

It runs perfectly. A few things of note. We have a tropical fan clutch which causes the engine to run cool. I've replaced the coolant temp sensor and the connector to it.

Any help is greatly appreciated!
HACK

[Edit: I just noticed Art posted while I was composing, so take note of his wisdom first, especially checking the main system ground point on the fuel rail.]

Did you also check port 6 in the diagnostic connector for ignition control module OBD codes? That could be very telling. It gets the same signal from the ECT and I would expect to see a 2-2-4 code. If that code isn't present then that says the problem is likely not wiring at the connector, but more subtle, such as a faulty ECT voltage that only upsets the ECU, not the ignition controller. In that case, check the sensor voltage as in Art's post.

OBD code 1-2-3 on port 2 says the ECU isn't getting a signal, leastwise not any kind of proper signal, from the ECT (coolant temp sensor, aka. block temp sensor). You more than likely still have a problem at the ECT that was replaced, either a wiring connection problem, the sensor not grounding to the block or the replacement sensor is faulty or incorrect. Check the wiring handywork there. Start by making sure the connector is properly seated.

Of note, that code can be raised if the ECT wiring is damaged and shorting to ground. Odds are that would be under the hood where work was being done on the connecter. You say the connector was replaced. Any chance it was wired backwards? The next likely place I can think of would be a damaged connector at the ECU with bent pins in the connector (pin 13) shorting to a neighbouring pin, such as from improper use of a meter probe into the connector (supposed to probe into the side slots).

You're likely aware there are two temp sensors under the intake manifold, so make sure you're checking the correct one. The forward one is for the dash temp gauge. The one at the back that's more difficult to reach is the coolant temp sensor for the ECU and ignition controller.

The usual symptoms for a bad ECT are idling rpm and running problems without an OBD code being set. So it's likely something fairly basic like wiring or the wrong sensor.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Thanks Art and Dave,

Oddly I went out to the car and removed the connector to the AMM then plugged it back in with the battery connected but without the key on.

Then looked at the covered wiring next to the firewall for broken wires but in honesty did little but disturb the wires. I don't believe I've ever touched the wires on the firewall. Never found it necessary.

I got in the car and started it. No light. I let it run for 30 minutes in our driveway then took it for a 15 minute drive..no light.

Not sure what happened. But I'm not complaining.

The only thing I can think of is a damaged or loose wire/ground?

Does disconnecting the AMM reset the system better than just removing the negative battery cable?

I'm going to get the car smogged then I guess I'll look for a damaged wire? When California cools down.

Many Thanks,
HACK

I wouldn't go celebrating just yet. You haven't really identifed either the problem or the fix, so be prepared for it to come back. If you head off to the inspection station to soon, odds are Murphy will be waiting for you there to turn the Check Engine light on again just as you pull up.

If things mysteriously changed after being under the hood, you likely disturbed a poor wire connection, perhaps a ground like at the fuel rail, or even a slightly loose pin in a connector, even a broken wire with the insulation still intact. Changed weather conditions like humidity and temperature can affect weak contacts, especially grounds, as can conditions under the hood.

Disconnecting the AMM doesn't clear OBD codes. It's simplest to clear the codes at the diagnostic connector after reading all the stored codes. Read them twice until all codes repeat to be sure you got them correctly. Remember to do both ports 2 & 6. You'll soon remember the simple procedure if you do it often enough. If you can't soon figure out the problem from the code, then reset the codes and wait for them to come back, either immediately or over the next few days. The condition that set the code may have been transitory and has now gone away. An example of a transitory condition might be an OBD fuel trim code set during extreme stumbling and stalling at very low rpm simply caused by incorrect idle conditions or loose ignition wires that have now been dealt with.

Disconnecting the battery for a moment also works. Many people prefer this method as it's easy to remember and clears everying at the same time, especially those people who don't use the diagnostic connector often enough and find it a nuisance. Note that disconnecting the battery will also clear settings like radio tuner programming, clocks and needing to re-enter radio codes, also resetting the ECU adaptive fuel trim program (no big deal, over a number of starts, like the next 16, it will have learned a full new set of adaptive program settings). Another way to reset codes without affecting clocks and radio settings is to interrupt power to the controllers using their fuses.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Thanks Dave,

All good points to remember and check.

I appreciate your time and expertise!

Best,
HACK

I've seen others report that same problem, where the ECT-pointing code isn't solved by changing the sensor. No confirmed causes, but several things to check:

1. Be sure the fuel rail grounds are in good shape. Each is a single wire to the eye under the bolt, but just up inside the vinyl sleeving is a splice to multiple things needing grounding.

2. Make sure you used the proper sensor; that you aren't replacing one identical to the wrong one your previous owner left you. It should have a black body, not a blue one.

3. As a last resort, monitor the voltage at the ECU connector starting with a cold engine and until it reports the DTC. Look for voltage corresponding to the numbers on the right hand side of this table.


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Art Benstein near Baltimore

Those who race through life finish first. (Darrel Hunsbedt)