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The temperature gauge on my 98 Mazda 626 2L MTX recently went all wonky, most of the time it's pegged low, but sometimes in just jumps around, but never above the actual temperature of the engine. After hooking up to the ECU and seeing it reporting a steady and accurate temperature, I realized that there is a separate single wire temperature sensor for the dash.

While shopping for a replacement I ran across this advertisement:

When not replaced, vehicle may experience inaccurate temperature gauge function, overheating engine, internal engine damage and/or excessive fuel consumption

This would seem to indicate that the data from the single wire temperature sender unit is also used as an input to the ECU, and if this sensor is bad could cause performance problems.

Is this in fact the case, or do these types of temp units just hook straight to the dash without providing input to the ECU?

I've heard wonky coolant temp sensors can cause odd performance problems.

I'm asking both for my specific car and in general.

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  • Not sure but I drove a 1989 Opel Vectra for a while without the separate dash temperature sensing unit (ok, the unit was there but the wire was broken). The car worked absolutely fine, but I didn't test it on hot summer days in traffic jam, so there perhaps can be risk of overheating due to the radiator fan not starting (not sure if the radiator fan is controlled by a separate temp sensor). However, I eventually repaired the broken wire so I could see engine temperature inside the car on the dashboard.
    – juhist
    Jul 18, 2017 at 17:21

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For your 626, I'm seeing two different temp sensors listed. I'm seeing this single bladed one:

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Which shows to be associated with the gauge. I also see this two bladed one:

enter image description here

Which I assume is for the ECU. I'm going to give an educated guess and state your Mazda is setup with the separate sensors which works out well in this case.

As for other cars, this is not an uncommon setup. I've seen a lot of different models setup either way. It all depends on how the manufacturer has them setup. I'd suggest it is going to be rarer in newer vehicles. This is because in a lot of them, the ECU runs the dash ... it collects the data from the different components and displays those readouts on the dash. In those cases, you'd only see one temperature sending unit and what you'd see on the ECU if plugged into it gather live data and what is displayed on the dash to be quite similar.

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  • So the dash temp sender ( single wire ) is unlikely to be providing additional data to the ECU beyond what the two wire sensor is providing?
    – Robert S. Barnes
    Jul 18, 2017 at 21:10
  • @RobertS.Barnes - Exactly.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 18, 2017 at 21:26
  • Ok, I was actually thinking it might be a good safety feature to have redundant sensors, but I guess I'm over thinking things.
    – Robert S. Barnes
    Jul 18, 2017 at 21:49
  • @RobertS.Barnes - I'd think redundant sensors would just cause issues, as they'd always be fighting with each other. Which would the ECU believe? Neither would have the same temperature at the same time. If you averaged them then what happens if one goes bad? It could get ugly quickly.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 18, 2017 at 23:11
  • I was thinking more along the lines of a reality check, like if one sensor shows a value way off what the other sensor show set a DTC. Don't allot of cars have both a crank and cam sensor and compare their values as a reality check against each other?
    – Robert S. Barnes
    Jul 19, 2017 at 6:50

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