Before changing big dollar items make sure fu8 is good. It feeds battery positive power to the power card.
Strangely I just had a OP30 that would code 2D then go right into 26 , changed out TM's , power card,leads after testing for shorts, etc. Thought I had it, came back, swapped motor from another OP that had been running a long time, still coded after a few days. Then I noticed that the truck was slow to start and Q1 didn't sound right even though it and Rect 1 tested good. I changed out Q1, & Rect 1 truck ran normal --for a few days. Code 26 came back, new motor and new power card finally solved it. I believe what happened was this, Q1 was not conducting pulses correctly at lower end causing the power card to pulse the field stronger to start the truck moving due to armature not responding that got the power card(maybe motor). The old motor I swapped from another OP shorted the fields when hot , took out the power card. I was chasing my tail, new motor and another power card solved it but sad case of "known good motor that wasn't" sure put a monkey wrench in the works!
the Q1 is in the armature circuit. a 26 would lead you to the power card, open field wires or TM1 or TM2 transistors.
JonG thanks thats what ist was thanks for your help. have you had any experience with a CAT EP20T code 50 steering sensor I have replaced and speed sensor and main board.
It's probably the drive motor or the cables to the motor. Code 26 is an open detected in the field circuit. It could lead to the Q1 circuit...