Would like to increase maximum temp to my spa. Have tried to add resistance to sensor adapter wires coming off thermistor (at input/output headers) to no avail. Using RP2100 raypack - 265K BTU heater with remote control panel. Please help.

sensor failure possible chemistry out of balance header leaking possible overheating check gas pressure also for blocked burner tube

Location of Intake Air Temperature Sensor:
The intake air temperature sensor, which is mounted on the mass air flow meter, monitors the intake air temperature. The intake air temperature sensor has a built-in thermistor with a resistance that varies according to the temperature of the intake air. When the intake air temperature becomes low, the resistance of the thermistor increases When the temperature becomes high, the resistance drops .

Thermistors are inexpensive, easily-obtainable temperature sensors. They are easy to use and adaptable. Circuits with thermistors can have reasonable outout voltages - not the millivolt outputs thermocouples have. Because of these qualities, thermistors are widely used for simple temperature measurements. They're not used for high temperatures, but in the temperature ranges where they work they are widely used. Thermistors are temperature sensitive resistors. All resistors vary with temperature, but thermistors are constructed of semiconductor material with a resistivity that is especially sensitive to temperature. However, unlike most other resistive devices, the resistance of a thermistor decreases with increasing temperature. That's due to the properties of the semiconductor material that the thermistor is made from. For some, that may be counterintuitive, but it is correct. Here is a graph of resistance as a function of temperature for a typical thermistor. Notice how the resistance drops from 100 kW, to a very small value in a range around room temperature. Not only is the resistance change in the opposite direction from what you expect, but the magnitude of the percentage resistance change is substantial.

SNS means the temp sensor is out of acceptable range. Could be the following:

1. Water temp is below 36 degrees F
2. The sensor contains 2 thermistors, if they are more than 2 degrees apart and SNS failure will occur
3.Loose or corroded connection at P1 on the main board
4. If the sensor has come out of the JACO fitting on the header, it may read a SNS failure.

Solution, visually inspect the connections for corrosion. If it looks ok, then replace the temp sensor.

Engine coolant temperature (ECT) sensor is a thermistor screwed into a coolant passage in the cylinder head. Sensor resistance is high when coolant temperature is low and resistance drops when coolant temperature increases. The powertrain control module (PCM) provides a 5 volt reference and a ground to the sensor. The PCM monitors voltage drop to determine coolant temperature. If the ECT reads less than freezing temp. when engine has been running for more than a few minutes, the PCM determines a circuit fault and sets this code. Or if the PCM determines the sensor resistance is out of specs, this code is set.

The reference values given below are measured between ECM terminal 72 and ground...

When measuring input / output voltage be careful not to use ECM ground terminals. The ECM will enter fail safe mode once a malfunction is detected and the MI will light up.

Usually, P0118 is indicative of a bad temperature sensor, but doesn't rule out these other possibilities. If you're unsure of how to diagnose a PCM, do not attempt.

Keep us updated.

This indicates the the 2 installed temp sensors are having a reading the is more than 2 degrees in difference. It is either a failed sensor, or one of the sensors is not fit snuggly into the header.
To test a sensor, turn your multimeter on OHMS or RESISTANCE and connect probes to each wire. at 80 degrees, the sensor should read 93 OHMS. At 70 degrees it should read 118 OHMS
at 60 degrees it should read 153 OHMS

Water temp sensor. Plugs into thermostat board and follow wires around to the main water header, there it should slide into the header. It will be a small tube connected to the wires. The new one will slide back into place, run wires back to the front of heater and plug back into board.

Part of the ignition system is bad. It is probably the ignitor, if you change it shut main gas supply to unit. Also, shut the power to the heater and turn it back on after repair

temp control for cooloing system or heater sysem?

ECT - Engine Coolant Temperature Sensor

The engine coolant temperature (ECT) sensor (Figure 27) is a thermistor device in which resistance changes with temperature. The electrical resistance of a thermistor decreases as the temperature increases, and increases as the temperature decreases. The varying resistance affects the voltage drop across the sensor terminals and provides electrical signals to the PCM corresponding to temperature.
Thermistor-type sensors are considered passive sensors. A passive sensor is connected to a voltage divider network so that varying the resistance of the passive sensor causes a variation in total current flow.
Voltage that is dropped across a fixed resistor in a series with the sensor resistor determines the voltage signal at the PCM. This voltage signal is equal to the reference voltage minus the voltage drop across the fixed resistor.
The ECT measures the temperature of the engine coolant. The sensor is threaded into an engine coolant passage. The ECT sensor is similar in construction to the IAT sensor.
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