Short version: A 2010 Prius kept setting code P261C after another shop replaced the electric water pump. The real problem was a broken signal wire at the pump connector feeding garbage data to the ECU. We repaired the wire, cleared the code, and it stayed gone.
A 2010 Prius with 202,000 miles came in with a check engine light that another shop couldn’t get rid of. They’d already replaced the water pump, but the code kept coming back. The customer brought us the paperwork from the other shop and asked if we could figure out what they missed.
What we found
We scanned and pulled P261C: engine coolant pump “B” control circuit low. Same code the other shop had been chasing.
The freeze frame data was the first clue. The water pump was spinning at a much higher speed than the computer was commanding. Looking at live data, we confirmed the pump was running even when the ECU wasn’t telling it to. The other shop had installed an AISIN pump (correct brand), so the part itself wasn’t the issue.
We started checking wiring at the pump connector and found a broken wire. One of the signal wires at the water pump connector had snapped. That broken wire was sending garbage data back to the ECU, causing the pump to run at full speed or come on when it shouldn’t. The ECU saw the mismatch between what it commanded and what the pump was actually doing, and set the code.
The other shop replaced the pump but never checked the wiring. Right part. Missed the actual problem.
The fix
We repaired the broken wire at the water pump connector, cleared the code, and test drove. No check engine light returned.
What this means for you
A fault code tells you which circuit has a problem. It doesn’t tell you which part in that circuit failed. P261C points to the coolant pump control circuit, and the most common fix is replacing the pump. But if the wiring between the pump and the computer is damaged, a new pump will have the exact same problem.
This happens more often than you’d think, especially on higher-mileage cars where connectors get brittle and wires fatigue from vibration. If you’ve had a part replaced and the code comes right back, the part probably isn’t the problem. Ask the shop if they checked the wiring and connector before they replaced anything. That question alone can save you from paying for parts you don’t need.
Related reading
- 2016 Prius “Head Gasket” That Two Shops Couldn’t Fix Was Really a Warrantied Catalytic Converter
- 2011 Prius Won’t Detect the Key Fob: Two Shops, Two Locksmiths, and the Real Fix
- 2007 Prius U0293: Why a New Steering Sensor Killed the Hybrid System
- Toyota Hybrid Trouble Codes: The Full Guide
About the author: Travis Decker is the owner of Atomic Auto in Portland, Oregon, and an ASE Master Technician (L1, L3). Atomic Auto specializes in Toyota, hybrid, and EV service.