The heater on my Toyota Previa is not working and the temp gauge is not moving even when the car has been running for a while. Please help

Question edited for clarity.
Question moved to model category.

Here are the Fuse Box Diagrams

Toyota Previa 1995 Pin Fuse Box/Block Circuit Breaker ...

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Apr 15, 2023 - Toyota Previa 1995 Pin Fuse Box/Block Circuit Breaker Diagram ; 1, Relay Heater ; 2, Relay Heater ; 3, (Headlamp) Relay Light ; 4, Relay Engine Oil.

Fuse Box Diagram Toyota

Fuse Box Diagrams https://fuse-box.info > Home > Toyota
Fuse box diagrams (location and assignment of electrical fuses and relays) Toyota Previa (1995, 1996, 1997). Toyota Prius (XW11; 2000-2003)...> ...

Toyota Estima Previa 2G fuse and relay

FuseAndRelay.com https://fuseandrelay.com > toyota > estima-previa

Designation of fuses and relays Toyota Estima Previa 2nd generation, locations of control units and their diagrams. Cigarette lighter fuse.

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The problem is most likely the air temp control blend door is inop due to being stuck or the motor drive is defective or both.

If all voltages checked ok,then most probably its faulty instrument cluster board on which gauges are connected.Yes if cluster circuit board looks ok,then also it can be faulty.Somewhere there is minute short or burn on board then such prolem arises.
Also check the fuse box.One under hood and other under dash.Inspect each and every fuse and see if all are fine or not.
---------------For more troubleshooting on these cluster gauges i suggest you to refer the list of help links mentioned below.Its step by step instructions will help you to confirm the exact cause of problem in these conditions:---------
Click the link below:------

2004 Toyota Tundra: Oil Gauge and Temp Gauge Not Working?

http://howtobyme.blogspot.in/2012/05/2004-toyota-tundra-oil-gauge-and-temp.html

--------------

2005 Lexus SC430: Fuel Door, Dome light and Fuel Gauge Fuse Location?

http://howtobyme.blogspot.in/2012/05/2005-lexus-sc430-fuel-door-will-not.html
----------

Toyota 4Runner: Temperature Gauge Sensor

http://technoanswers.blogspot.in/2012/04/toyota-4runner-temperature-gauge-sensor.html
------------

Temperature gauge sits over half way mark?

http://schematicsdiagram.blogspot.in/2012/04/temperature-gauge-sits-over-half-way.html

---------------

Heater blows intermittently and temp gauge goes erratic?

http://technoanswers.blogspot.com/2012/01/heater-blows-intermittently-and-temp.html

----------

Temperature gauge does not move and no heat when heater is ON?

http://technoanswers.blogspot.com/2012/01/temperature-gauge-does-not-move-and-no.html

-----------

2003 Pontiac Grand Prix: Erratic Fuel Gauge?

http://technoanswers.blogspot.in/2012/06/2003-pontiac-grand-prix-erratic-fuel.html

-----------

Temperature gauge keeps fluctuating up and down close to red?

http://technoanswers.blogspot.com/2012/01/temperature-gauge-keeps-fluctuating-up.html

-------------

Temperature gauge on the dash quit working on Pontiac Grand Prix?

http://technoanswers.blogspot.com/2012/01/temperature-gauge-on-dash-quit-working.html

---------------

Fuel gauge jumps from empty to full?

http://schematicsdiagram.blogspot.com/2011/12/fuel-gauge-jumps-from-empty-to-full.html

--------------

Gas gauge stuck on empty on 2002 Mercedes Benz 240c?

http://repairhelpcenter.blogspot.in/2012/02/gas-gauge-stuck-on-empty-on-2002.html
---------------

• Oldsmobile Temperature Gauge Lamp Switch?

-----------------

Fuel gauge drops to Empty?

http://technoanswers.blogspot.com/2012/01/fuel-gauge-drops-to-empty.html

----------------

Dodge neon gauges not working?

http://technoanswers.blogspot.in/2012/02/dodge-neon-gauges-not-working.html

------------

Temperature gauge went up?

http://schematicsdiagram.blogspot.com/2011/12/temperature-gauge-went-up.html

------------

Temperature gauge stays At cold on jaguar?

http://technoanswers.blogspot.in/2012/02/temperature-gauge-stays-at-cold-on.html
-----------

How to replace Instrument panel and dashboard on Toyota car?

http://repairhelpcenter.blogspot.in/2012/02/how-to-replace-instrument-panel-and.html

-----------

How to remove instrument panel on Chevrolet car?

http://technoanswers.blogspot.in/2012/02/how-to-remove-instrument-panel-on.html

-----------

How to remove control panel on Buick regal?

http://howtobyme.blogspot.in/2012/02/how-to-remove-control-panel-on-buick.html
---------------
These details will help.Thanks.

If all voltages checked ok, then most probably its faulty instrument cluster board on which gauges are connected. Yes if cluster circuit board looks ok, then also it can be faulty. Somewhere there is minute short or burn on board then such problem arises.

Also check the fuse box. One under hood and other under dash.

Inspect each and every fuse and see if all are fine or not.

---------------

For more troubleshooting on these cluster gauges I suggest you to refer the list of help links mentioned below. Its step by step instructions will help you to confirm the exact cause of problem in these conditions: ---------

Click the link below: ------

2004 Toyota Tundra: Oil Gauge and Temp Gauge Not Working?

http://howtobyme.blogspot.in/2012/05/2004-toyota-tundra-oil-gauge-and-temp.html

--------------

2005 Lexus SC430: Fuel Door, Dome light and Fuel Gauge Fuse Location?

http://howtobyme.blogspot.in/2012/05/2005-lexus-sc430-fuel-door-will-not.html

----------

Toyota 4Runner: Temperature Gauge Sensor

http://technoanswers.blogspot.in/2012/04/toyota-4runner-temperature-gauge-sensor.html

------------

Temperature gauge sits over half way mark?

http://schematicsdiagram.blogspot.in/2012/04/temperature-gauge-sits-over-half-way.html

---------------

Heater blows intermittently and temp gauge goes erratic?

http://technoanswers.blogspot.com/2012/01/heater-blows-intermittently-and-temp.html

----------

Temperature gauge does not move and no heat when heater is ON?

http://technoanswers.blogspot.com/2012/01/temperature-gauge-does-not-move-and-no.html

-----------

2003 Pontiac Grand Prix: Erratic Fuel Gauge?

http://technoanswers.blogspot.in/2012/06/2003-pontiac-grand-prix-erratic-fuel.html

-----------

Temperature gauge keeps fluctuating up and down close to red?

http://technoanswers.blogspot.com/2012/01/temperature-gauge-keeps-fluctuating-up.html

-------------

Temperature gauge on the dash quit working on Pontiac Grand Prix?

http://technoanswers.blogspot.com/2012/01/temperature-gauge-on-dash-quit-working.html

---------------

Fuel gauge jumps from empty to full?

http://schematicsdiagram.blogspot.com/2011/12/fuel-gauge-jumps-from-empty-to-full.html

--------------

Gas gauge stuck on empty on 2002 Mercedes Benz 240c?

http://repairhelpcenter.blogspot.in/2012/02/gas-gauge-stuck-on-empty-on-2002.html

---------------

• Oldsmobile Temperature Gauge Lamp Switch?

-----------------

Fuel gauge drops to Empty?

http://technoanswers.blogspot.com/2012/01/fuel-gauge-drops-to-empty.html

----------------

Dodge neon gauges not working?

http://technoanswers.blogspot.in/2012/02/dodge-neon-gauges-not-working.html

------------

Temperature gauge went up?

http://schematicsdiagram.blogspot.com/2011/12/temperature-gauge-went-up.html

------------

Temperature gauge stays At cold on jaguar?

http://technoanswers.blogspot.in/2012/02/temperature-gauge-stays-at-cold-on.html

-----------

How to replace Instrument panel and dashboard on Toyota car?

http://repairhelpcenter.blogspot.in/2012/02/how-to-replace-instrument-panel-and.html

-----------

How to remove instrument panel on Chevrolet car?

http://technoanswers.blogspot.in/2012/02/how-to-remove-instrument-panel-on.html

-----------

How to remove control panel on Buick regal?

http://howtobyme.blogspot.in/2012/02/how-to-remove-control-panel-on-buick.html

---------------

These details will help.

Thanks.

Check out related help links with solutions to troubleshoot the problem:---
Click the link below:---
2004 Toyota Tundra: Oil Gauge and Temp Gauge Not Working?

http://howtobyme.blogspot.in/2012/05/2004-toyota-tundra-oil-gauge-and-temp.html

---------
Toyota 4Runner: Temperature Gauge Sensor http://technoanswers.blogspot.in/2012/04/toyota-4runner-temperature-gauge-sensor.html

-----------
Temperature gauge sits over half way mark? http://schematicsdiagram.blogspot.in/2012/04/temperature-gauge-sits-over-half-way.html
----------
Heater blows intermittently and temp gauge goes erratic?

http://technoanswers.blogspot.com/2012/01/heater-blows-intermittently-and-temp.html
------------- No heat on Oldsmobile Silhouette? http://technoanswers.blogspot.in/2012/02/no-heat-on-oldsmobile-silhouette.html
-------------
Temperature gauge does not move and no heat when heater is ON? http://technoanswers.blogspot.com/2012/01/temperature-gauge-does-not-move-and-no.html
----------
Temperature gauge keeps fluctuating up and down close to red? http://technoanswers.blogspot.com/2012/01/temperature-gauge-keeps-fluctuating-up.html
----------- Temperature gauge on the dash quit working on Pontiac Grand Prix? http://technoanswers.blogspot.com/2012/01/temperature-gauge-on-dash-quit-working.html
-----------
Dodge neon gauges not working? http://technoanswers.blogspot.in/2012/02/dodge-neon-gauges-not-working.html

---------
Heater blows cold? http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html

----------
Heat blows cold on drivers side and hot on passenger side? http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html

-----------
1992 Lexus LS400: Engine overheats, Antifreeze out of reservoir and heater Blows Cold Air? http://whatisbyme.blogspot.in/2012/05/1992-lexus-ls400-engine-overheats.html

-----------
These details will help.
Thanks.

You may need to replace your thermostat or heating coil.

bad ignition switch. replace and recheck

find the wire that goes to temp sender and apply earth to it turn ignition on the temp gauge should read hot if it does replace water temp sender if gauge dose not move gauge needs replaging

Previa heaters are usually ultra-reliable.
So, regarding your question, the only reason for the temp gauge to drop down with the heater on is the thermostat is sticking open, or you have a lower than recommended thermostat installed. The factory thermostat is a 192 degree F/89 degrees C stat. Down your way, a lot of the parts places vend the "summer" thermostats, and I suspect that this is your problem.
Look to see what the temp degrees that are printed on the stat. Should be 192 F or 89 C. The "summer" stats are 185 F/ 85 C. Also, make sure that your stat has a small "wiggle valve", some aftermarket stats do not have this. Very important. Install the stat with the "wiggle valve" pointing straight up/12 o'clock...
Regarding your trans dropping out od OD, this is because the coolant temp is dropping below 172 degrees F. This is the threshold for the OD to engage. Below 172/no OD engagement.