What the C155 alarm means
With servos on, the robot drifts or cannot hold its position; the controller suspects an incorrectly configured payload and stops. This is a high-frequency ticket for our Universal Robots repair team, and on CB3 / e-Series (UR5/UR10/UR16/UR20) systems one inspection usually identifies the fault.
Most common causes
- Payload mass or center of gravity set wrong
- Gripper/tool weight changed (empty vs full) without updating the payload
- Brake wear on a heavily loaded axis
- Mounting orientation configured incorrectly
Step-by-step field fix
- Re-measure the real tool weight and center of gravity
- Update the payload in the active installation
- Use payload switching programs if the tool grips parts
- Test brake holding by enabling servos at the affected pose
- Replace worn brakes if drift persists with correct payload
How we diagnose motion alarms on site
Universal Robots c155 belongs to the motion & path family. On Universal Robots c155 the failing stage is joint-integrated drives (motor, drive and harmonic reducer in one joint) on the UR control box (CB3 / e-Series) - the gap between command and execution: trajectory planning, joint limits, singularities, following error and mechanical resistance. We start reading Universal Robots c155 at the PolyScope event and alarm pages; codes carry a C prefix (C15x, C5x, C7x).
The chain involved in Universal Robots c155 runs: program command -> trajectory planner -> joint targets -> servo execution -> actual position feedback -> deviation compare.
If Payload mass or center of gravity set wrong on Universal Robots c155 comes with a feeling of drag, the direction is clear: Following error that stays high on one axis only is usually rising mechanical resistance, not a weak servo. We turn the axis by hand with the brake released to feel smoothness, and listen to the reducer for once-per-revolution knocking - a single tick per turn on an RV reducer usually means internal bearing or cycloid damage. Grease that is black, dry or carries metal powder is the plainest evidence. Left alone, this graduates from a motion alarm to a scrapped reducer.
Field notes on this page cover Universal Robots c155 (Protective Stop — Robot Cannot Maintain Position); written by the Henghuan repair team, updated 2026-10-01.
Real Field Case
Machine: UR UR5e (2019)
Symptom: A UR5e raised C155 (robot cannot hold position) and drifted down slowly while idle.
What we did: We judged insufficient brake holding force on the affected joint, inspected and serviced the brake, then re-ran the hold test.
Result: It held position properly after the fix with no further drift; about 3.5 hours.
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Related Motion & Path alarms
C150 — Protective Stop — Position Close to Joint Limits C153 — Protective Stop — Position Deviates from Path C154 — Protective Stop — Position in Singularity C157 — Protective Stop — Collision DetectedFrequently asked questions
QWhat does the Universal Robots c155 alarm mean?
With servos on, the robot drifts or cannot hold its position; the controller suspects an incorrectly configured payload and stops. This is a high-frequency ticket for our Universal Robots repair team, and on CB3 / e-Series (UR5/UR10/UR16/UR20) systems one inspection usually identifies the fault. Universal Robots c155 sits in the motion & path family; on our work orders the highest-frequency trigger for this code is "Payload mass or center of gravity set wrong".
QCan I clear a Universal Robots c155 alarm myself?
Whether Universal Robots c155 clears on its own depends on the outcome of "Re-measure the real tool weight and center of gravity". Finish the flow with "Replace worn brakes if drift persists with correct payload" to confirm Universal Robots c155 - do not skip it. If that step is clean, the second most frequent cause on Universal Robots c155 is "Gripper/tool weight changed (empty vs full) without updating the payload". Two attempts and still there means stop: Universal Robots c155 involves the arm working envelope, and running Universal Robots c155 on can turn a repairable fault into a replacement.
QFor motion alarms like Universal Robots c155, can you triage remotely first?
The commonest causes we see on Universal Robots c155 are "Payload mass or center of gravity set wrong, Gripper/tool weight changed (empty vs full) without updating the payload, Brake wear on a heavily loaded axis", so we stock the van against those. For Universal Robots c155 specifically, we carry mechanical and configuration items: grease, limit switches, drag-chain segments, encoder cables and the parameter sheets for common reducers. For Universal Robots c155, motion alarms can often be narrowed remotely first, so an on-site visit is not always immediate. Before we roll, tell us what Universal Robots c155 shows on the PolyScope event and alarm pages; codes carry a C prefix (C15x, C5x, C7x) and what the machine was doing - phone or video triage is free.
This page on Universal Robots c155 is based on the field experience of the Henghuan Robotics repair team. Alarm explanations are our own summary for troubleshooting reference and may differ across controller and software versions — if unsure, verify against Universal Robots documentation or reach out to us. Henghuan is an independent service provider with no affiliation or endorsement by Universal Robots.