I have had this car for 2 years now. After about 8 months I noticed the previous owner (or more likely his mechanic) smashed the check engine bulb, while I was fixing the odometer gear. Code 123 keeps coming up but not all the time. I usually comes on after a long highway run. I thought I had fixed it but its back with a vengance.

At this point the temp sensor has been changed 3 times and all of the old sensors tested normal after removal. I have tried 3 ECUs (951, 561, and a EGR version)and all throw the same code. All three ECUs work fine in my 89. I have replaced the temp sensor wiring harness from the sensor to the firewall. I have cleaned and checked the manifold grounds. I have tested the resistances over and over again at the ECU plugs.

Today the light came on after about 2 hours of highway driving. I stopped checked the code and cleared the ecu. The light came back on after about 2 minutes. When I got home I measured the resistances at the ECUs. They were both .21 ohm, which is low. The usual failure for temp sensors is high resistance, so I am wondering if I should believe the sensor. Could I have a hot spot in the head?

I did just replace a severely warped head because the previous owner used sealer on the headgasket that blocked some of the cooling passages. Does it make sense that some of the passages may still be blocked?

Does the ECU go into limp mode when it has a 123 error? I am wondering because the car is getting over 30mpg even with the light on.

Its getting to the point that I don't know what to do? I have spent many hours on it, as well as several very experienced Volvo mechanics.

sorry for the long post, but any suggestions would be much appreciated.

I've had two 1990 240s with this problem - my answer was to get the OEM sensor from FCP rather than the crap aftermarket ooes I had put in both of those cars.

Other posters here have great suggestions for troubleshooting but I thought I'd throw this one in just in case.
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Bad Blue - '86 245 with 303,000 mi. (now for parts), Good Green 1990 245 with 178,000 mi., Blacky - '91 740 wagon with 180,000 mi., Mean Green - '94 850 with 222,000

hello i have the exact same issue. cannot get rid of it. comes back immediately. did it cause your car to start hard? mine is starting and running well after about five to ten cranks and tries.

Hi Rhino Volvo,

What year 240 is yours? I'll guess at least a 1989 and newer? How many miles? How old is the coolant? Have you performed any work on the engine cooling system (radiator, engine coolant replacement (antifreeze + distilled [never tap] water, thermostat, hoses).

You searched for and replied to a thread dated from year 2008.

You have checked the OBD-1 socket 2 (fuel injection and connected emissions sensors and devices). Have you checked exhaustively any other codes in socket 2 and also socket 6 (ignition and connected emissions sensors and devices).

See:

Engine and OBD Diagnostic Codes - https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm


Codes for LH-Jetronic (fuel) 2.4 / 3.1, EZK (ignition) 116.

Code 1-2-3 is an engine coolant sensor fault. The on-dash orange Check Engine indicator illuminates.

The ECT sensor is two engine coolant sensors in one: One sensor for the fuel injection ECU and the other for the ignition ECU. So, checking socket 6 OBD may help with the diagnosis. Repeat OBD check in the event the ECUs in your model year Volvo 240 have other fault codes stored.

The fault may be the ECT sensor itself or the engine control wire harness between the ECT and the engine control units (ECU). Includes all connectors at the ECT and the ECU and in between.

So, with your trusty voltmeter, you can test the sensor wiring. This board is replete with the procedure in various forms to do so. May be nothing more than, with battery disconnected, a disconnect, reconnect on the wire harness connectors, or may involve ECT replacement. ECT replacement is easier, or best, with the intake manifold removed. (Some are able to replace the ECT without air intake manifold removal. As seen on a You Tube video or two with a tool set-up.)

See line art wiring diagrams here for your model year Volvo 240:
http://www.volvowiringdiagrams.com/?dir=volvo/240%20Wiring%20Diagrams

The ECT sensor is somewhat difficult to reach. You may need remove the air induction piping under the throttle body to get at the connector. And even then it will be difficult. Small hands help.

Or, you can, at the ECU connector, back probe the specific pin positions and perform a continuity and resistance check.

Other members can provide you guidance as to diagnosis using fewer words.

You are in TN-state? Have much rust / corrosion about the engine bay?

Hope that helps get the help to you started.

Earl Grey Tea Boy with hunny and milk.

Hi Jessie,

Here's a bump up anyhow. FWIW, this is what the Regina (essentially the same as LH2.4) has to say about the 123 code (paraphrased):

"If the ECT signal indicates below –40C or above 120C the fault code [and CEL] is set.

The ECU then assumes 20C at startup and approx. 90C at all other times (limp home) so the car can still be driven."

The ground path is thru the sensor threads & body, so there shouldn't be any tape or teflon goo on the threads, just a crush washer for sealing. The Bentley shows the common ground for both ECUs to be the one on the left (fwd) fuel rail bracket.

P.S. Just curious, are you getting any EZK codes in 6? It can also set the 123, but doesn't add the CEL, and just defaults to an assumed 90C all the time. A ground side fault in the sensor could affect both CU signals it seems like.

I actually didn't check the ezk this time but I have never got a code from the ezk in the past.

Thanks for the info on the limp mode. At least the car should get reasonable fuel economy when the light is on. It does not make sense that it is a ground problem because I am getting low resistance not high.

I am starting to think its not the temp sensor, but another warped head that is starved for coolant around the temp sensor. I drove it today on short trips and the light stayed off. It seems to only happen after a long highway drive on hot days. There are other signs of a warped head. Oil is leaking out and I don't think it is the valve cover. It also runs a little bit rough when it is cold but smooths out as it heats up. No white cloud of smoke when you start it, like the last one, but I suspect I will start to see that this winter when it gets colder.

I'm not sure what I am going to do. I don't know if it worth putting another head on this engine. I am thinking it might be better in the long run to put in a good used motor rather than keep messing with this one.

Do you have a coolant leak, then?

How is the coolant level in the coolant expansion tank? Never allow coolant level to get so low.

This forum is full of symptoms and suggestions for coolant leaks, oil leaks, leaking cylinder head gasket, faulty coolant systems and such.

If the engine is leaking oil, is the PCV system clear and breathing. A clogged PCV systems pushes oil out lower seals first, yet can push engine oil out other places like the seal at the back of the cylinder head plugging the hole between the valve cover and the top of the cylinder head.

Perhaps use a manual car wash to wash away engine oil and grime to get a clean start and inspect for coolant / oil loss.

Just a thought:
There are 2 different temp sensors, one for the guage and one for the ECU. The one for the ECU is more to the rear. Possible you are changing the front sensor?
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David Hunter

Thanks, but no the guage sensor is a single wire sensor.