Diesel exhaust gas temperature sensors can seize, break off and sometimes damage the threads in their mounting boss during removal preventing the installation of a new EGT sensor rendering the diesel oxidation, diesel particulate or selective reduction catalysts useless. We have some repair advice to offer you from the best method of removing the sensors to repairing or replacing the mounting boss.
So you have a diesel truck in the shop that either won't start or has a check engine light on due to an EGT sensor concern. Your first step is to diagnose the concern and determine what the proper and effective repair will be. If an EGT sensor requires replacement, first inspect the entire exhaust system for damage including the tailpipe for the presence of black soot and other sediments or fluid that would indicate that the DPF is cracked or damaged in some way that it would require replacement to begin with.
Remove The EGT Sensor
As per Ford Motor Company and the experience of service technicians, the best method for removing these sensors is as follows:
- Cut off the wire from the top of the sensor to allow a deep six point 13mm socket to be placed onto the sensor hex nut.
- Using an oxyacetylene torch carefully heat the sensor mounting boss until it glows bright orange.
- Using a breaker bar place the socket onto the sensor hex nut and loosen - allow the parts to cool.
- Inspect the sensor boss for damage - sometimes the sensor refuses to break loose, the nut beaks off, the threads will lock up after a few turns or the sensor will come out but the threads will be damaged. Depending on the results of your inspection the next steps may or may not apply.
Repair Damaged Threads
If the sensor comes out and the threads are damaged they can usually be repaired. Using a 12mm X 1.25 bottoming tap Identified by the flat tip shown in the picture the threads can be re-cut allowing a new sensor to be installed. Do not use a thread chaser/repair tap as those tools are not meant to cut the threads but only clean them up and will not be effective. When cleaning the threads with the tap use caution to avoid damaging the EGT sensor sealing surface at the bottom of the mounting boss.
Replace The Mounting Boss
The following procedure requires the use of an oxyacetylene torch and a welder. You must also obtain a new sensor boss. This repair is not recognized by Ford Motor Company and is not covered by any warranty provision and the parts other than the sensors are not available from Ford. There are sources for these bosses but you have to search for them:
- This source on EBAY will get you the right part.
- Search the Internet for "Exhaust Sensor Boss" or "Exhaust Sensor Bung" with 12mm X 1.25 threads
Step 1 - Access the damaged sensor mounting boss either by raising the body on chassis cab trucks with a dump body or removing the catalyst from the truck to work on a bench. In some cases you may be able to simply disconnect the inlet of the entire aftertreatment assembly and disconnect the mounts from the frame and drop the assembly down enough to provide ample room to work.
Step 2 - Heat the damaged EGT mounting boss with an acetylene torch until it glows bright orange, then twist it off with a pipe wrench.
Step 3 - Once removed you need to clean the area around the hole down to bare metal in preparation for welding.
Step 4 - Center the new sensor mounting boss on the hole and weld in place using an Arc-welder or preferably a Mig-welder. Remember that you are welding to a thin steel exhaust component, not a battle ship and excessive power and heat is not needed. This can be done by brazing just as effectively.
Step 5 - Reinstall the exhaust into the vehicle, apply nickel anti-seize compound to the new sensor fitting threads, install the sensor and torque to specification (22 lb-ft / 30 Nm) then apply silicone dielectric grease to the sensor electrical connector pins and connect the sensor to the vehicle harness. Ensure that the connector and the wiring is properly secured preventing contact with the hot exhaust system or other moving parts under the vehicle.
Article and Images by Keith Browning and Jeff Eichen