Skoda Laura engine temp issue: Could a faulty ECM be the culprit

Wouldn't a faulty ECM not allow the car to start in the first place? I'm suspecting that the engine coolant temperature sensor might not be functioning properly.

BHPian LoneRider recently shared this with other enthusiasts.

Hi BHPians,

I own a Skoda Laura Elegance 2011 2.0 Tdi Automatic and have started facing some issues with my car as below:

• When I turn the ignition to "ON" (car not started) position, the check engine and high coolant temperature signs get lit in the instrument cluster and the temperature gauge in the instrument cluster jumps to high and the car starts making a beeping noise. Simultaneously, the radiator fans start running at full speed (the car is still not started with just the ignition to "ON" position).
• The car, however, manages to start after 3-4 cranks. The moment the car starts, the temperature gauge drops to "C" and stays there forever no matter how long I drive the car. And the radiator fans continue to run at full speed constantly.
• When I stop the car, the fans continue to run for about 10 minutes after the key is removed and then they stop.
• The car then starts without hesitation till the engine is hot. However, once the engine becomes cold, the entire process repeats again.

Additional information

First I thought it is an issue with the instrument cluster since it had gone blank all of a sudden while driving. However, it came back up in seconds with the temperature gauge stuck to "C" and the fuel gauge stuck at "Full" (it was a tank full of diesel in there). I drove for about 160 kilometers and noticed the fuel gauge still sitting at "Full". I removed the negative terminal of the battery to reset the ECU. The fuel gauge was back to normal, however, the check engine light and temperature warning light remained as it were. I got my instrument cluster checked and all is fine.

Showed it to a mechanic and he says ECM is faulty and needs to be replaced. I am not convinced here since a faulty ECM would not let the car start at all. The mechanic is just trying to fill his pockets real deep.

After a bit of browsing, I am suspecting the engine coolant temperature sensor to be the culprit and planning to get that replaced.

Please, can you experts give your views as to what could be wrong as I don't want the mechanics to unnecessarily dupe me for simple work and charge me a heavy amount.

One more help please, would anyone know how many sensors are present in Skoda Laura for engine temperature and where they sit under the hood?

Here's what BHPian vishy76 had to say about the matter:

I might be the bearer of bad news here, but it does seem to be the ECM. I have noticed this happening in a Jetta TDI DSG too and a friend's Laura TDI, apart from a Passat B7 TDI on YouTube.

The problem arises when there is a short in the coolant circuit track on the ECM PCB, which causes the ECM to think the engine temp is always 150+ in spite of getting correct readings from the temperature sensor. The ECM has a fail-safe mechanism where it will disable the starter and cut fuel off if the engine is overheating. Hence the car doesn't start. This is also the reason why the radiator fans keep running at full clip.

The ECM can be repaired in most cases though by a specialist for less than 10-12k. You can swap the coolant temp sensor with that of another car just to be sure it is the ECM, but I would definitely blame the ECM only.

P.S. - You can also see the location of the engine temperature sensor in this video. As I said, swap it with a known good one and see if it remedies the issue just to be doubly sure.

Here's what BHPian Y@SH had to say about the matter:

Hi,

Get it checked at Select Auto, Dadar by Sunil. I had an ECU issue on my 2012 Skoda Laura, too. The car would not start sometimes and sometimes it would magically start.

No one was ready to repair it, others said the value of the car is lesser than the cost of a preowned ECU! Got it checked by Sunil and he got it repaired for about Rs 25,000.

Give him a call and give my reference. Let me know if you need any help.

Here's what BHPian pta320 had to say about the matter:

Please try to get an OBD scan of the car (costs 300-500 Rs) from any nearby FNG (make sure they are using a high-quality OBD scanner from Launch etc.) and let me know what fault codes, if any, are there. Also, the OBD device can pull up the live data so check the coolant temperature also and let me know. Also, take a snapshot of every live data while driving and while idling.

Without the fault code and actual live data, it's difficult to pinpoint what's wrong. It could be a bunch of issues like a bad thermostat, bad coolant temp sensor, bad connection, and the worst of all bad ECM.

Usually the fan cuts in when the coolant temperature reaches above 97-degree celsius. However, it's unlikely for that to happen on starting the car for the first time which points that the temperature readings are bad. But that's a very basic explanation, the real problem will be diagnosed only after an OBD scan. Also, OBD scanners (good ones from Launch) are able to factory reset the ECU, so also do that. Also, test the car with a different battery in case yours is going weak. If ECU isn't getting the proper voltage, then also such issues can come up.

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