Before I replaced mine, here is what the symptoms of a worn steering rod were:
When you are taxiing on the ground, the plane will have a tendency to move towards the side of the plane with the stronger steering rod. Most likely, this would be towards the left. The reason it would go to the left is the right rudder pedal is used more than the left (take-off, climbing, etc.), and it tends to wear out the right steering rod first.
The other effect I noticed was when on the roll out after landing. Once the plane had slowed down enough that the rudder was no longer effective, the plane would slowly start to pull to the left. I either had to put the brakes on or put in a little throttle (to get some airflow over the rudder) to straighten the plane on the runway.
For a long time I had mistaken this effect for a bad landing on my part, especially during crosswind landings. Finally, one calm morning I went out specifically to improve my landings to stop this problem. After two or three landings where the plane always veered in the same direction, I realized it was the plane, not me. I then took it to my mechanic and we spent about an hour checking all the rigging before we determined that it had to be the steering rods. I replaced both of them with the Macfarlane ones.
By the way, after removing the bad one, I pushed on the rod to see how weak the spring was, and it was still surprisingly strong. It just wasn't as strong as the other one, so the plane still turned to the left.
I would be leery about changing them with some used ones, based on my experience. You may find uneven spring tension and have ground control problems after you replace them.
