Thermocouple interference

Hi, would an AC Voltage of 15-20VAC, on a pair of type J thermocouple leads influence its mV output?

It depends. Are the wires run parallel to each other in the same conduit instead of just crossing perpendicular to the alternating source? What are you reading the thermocouples on? What are the readout's specifications for noise rejection? Have you looked at the thermocouple's output on a oscillascope (i.e. do you see a sine wave/ what makes you think it is a problem?) Does the reading on the thermocouple change at a steady state temperature condition if the 15-20 VAC source is shut off?

Hi. Im not sure how the power cables are run besides the thermocouple leads. the thermocouple is in a duct which is insulated, which is used to measure the temperature of our fuel line which has elements in. to maintain the fuel temperature. so basically the thermocouple is connected to a stand alone controller which switches the coil voltage to bring in the elements.the current temp reading is 15 deg and has 15-20 VAC on both the positive & negative leads,with respect to earth and that doesn't seem correct to me. i tried bridging the negative lead to earth, that got rid of the VAC to +- 200mV but controller still indicates 15 deg of the fuel line. i simulated a mV output into controller, and it works fine.

one more thing, the other thermocouples are working fine with no interference on them.

Unfortunately i can't remove thermocouple from the line because of its installation to test it on an oscilloscope and i don't know the noise rejection spec either, i have never experienced this before.
I initially thought that because there is a lot of interfere on the leads, its suppressing the mV output, therefore wrong indication of 15 deg, bt when i got rid of that interference, it remained the same.