Lotus Elan Central
Coolant Temperature Sensor Problems

Coolant Temperature Sensor

This sensor in the cooling system monitors coolant temperature for the ECU. Using the input, the ECU enriches the fuel mixture for cold starting and cold running. It also adjusts ignition timing, EGR flow-rate and the air/fuel ratio as a function of engine temperature. Never confuse the coolant temperature sensor with the coolant temperature sender which operates the temperature gauge. These are different sensors for different purposes, and if there is a problem with one of them, the other sensor is independent.

Coolant Temperature Sensor image 1

Common Problems:
1. Poor electrical connections.
2. Sensor corrosion from neglected coolant or improper mix.

Troubleshooting Tips:
1. Check and clean connections.
2. Check thermostat for correct temperature range, and proper function.
3. Replace defective sensor to correct poor performance and fuel economy.
4. Replace coolant that’s older than two years or 20,000 miles


I have had my CTS go bad on me twice before. The first time I couldn't figure out the problem as it came on rapidly without any warning light. I checked all I could think of but could find nothing wrong; I had to have the car trucked to the dealer. This was a costly fix for a cheap part! The second time the sensor acted up I figured out what it was right away, was able to make the diagnosis, and fixed it without spending a penny!! My goals here are to explain the functioning of the sensor, help you diagnose when it malfunctions, and help you locate the sensor and fix the problem!

CTS Function

The CTS is one of the many sensors the ECU uses to manage the engine. It sends the coolant temperature to the ECU at all times. The CTS is simply a resistor that changes its electrical resistance over the temperature band. High sensor resistance tells the ECU the coolant is cold, while low sensor resistance tells the ECU the coolant is hot.

A faulty electrical connection, such as at the sensor wiring contact, will act like an "infinite resistance" as the electrical loop is not complete--this will make the ECU think the coolant is as cold as the lowest it is programmed to read, which is -70 degrees C. A broken sensor will do this as well. The first time I had a problem with the CTS, the dealer put the Tech 1 tool on the car and instantly saw that the CTS was reading -70 degrees---> problem found immediately!! Too bad we do not yet have the home laptop program to interface with the ALDL or you could do this simple diagnosis at home or even on the road!!

coolant temperature sensor image 2


The CTS sits atop the Thermostat housing. It is directly behind the CAS and between the Crankcase Breather Oil Separator and the engine, near where the spark plug wires disappear beneath the cam cover lid. See the Engine Component chart for locations and drawings of what the parts look like!

The CTS is one of three coolant-related sensors, so do not get them mixed up (see picture below). The Temperature Gauge Thermal Switch sits directly in front of the CTS; it is the sensor that runs the dashboard temperature gauge. The Radiator Fan Thermal Switch sits on the other side of the thermostat housing from the other two; it is the trigger that tells the accessory radiator fans when to activate.

coolant temperature sensor image 3


When the CTS acts up [if you are lucky] you will get an engine light on the dash shortly after starting the engine. If you check the code with the ALDL you will get code "15." This should be your trigger to check the CTS immediately as the problem. When the sensor contacts go bad, the engine will perceive a coolant temperature of as low as -70 degrees. It will respond by opening up the Idle Air Control Valve (IAC) and running the engine mixture at full rich! You will notice the engine idle speed will be high from all the extra fuel. You should be able to smell all the extra exhaust fumes from the unburned fuel. Also you will find the engine to be less responsive to the throttle and perhaps even sluggish and quirky! All of these things happened to me the first time; when they happened again I knew what the problem was.

If these symptoms happen to you and you don't get a check engine light, it is still possible you have the same problem. I did not ever see a check engine light the first time it happened to me. All the symptoms were the same, and without the code there was nothing to guide me to this sensor as a cause. This is one of my inspirations for making this write-up--to help your awareness for this problem.

The Fix

This sensor is just behind the CAS and is hard to reach. [You may want to print the location map to help you find it and distinguish it from the other sensors in the vicinity!!] You must first release the little clip lock tab on the connection and then you can pull off the electrical lead to the sensor--you will (hopefully) find it is corroded! Then you have to do is clean up the contacts, either by scraping them with a small screwdriver or piece of sandpaper or similar. Then see if you can get some electrical contact protector spray and spray them before re-connecting to avoid the problem in the future.

This sensor connection points straight up and I believe it is for this reason that it is prone to have water standing on it if any gets into the area (such as engine detailing or very wet climates). It has a rubber seal at the connection but the water still gets in and the seal probably helps keep the water trapped in the contact area, which causes the corrosion to happen. I think the solution is that if you ever get water in the area as by cleaning or other obvious way, you should be sure to run the engine and warm it up so all the water evaporates.

If the contacts look clean I would still recommend "freshening" them and reinstalling the wiring to the sensor and restarting the car to see if the trouble has cleared. If the contacts are not the problem then you may have to assume the sensor is "bad" and replace it! Luckily, the actual sensor is not too expensive (around $20-30) and you can get it from an Isuzu dealer if you have access to one. Replacement is as easy as unscrewing the old one and putting in the new one. You may want to use some "sealant" on the threads to be sure it installs water-tight as it is in the coolant line which is pressurized when hot!

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