Rheem P1 code: not enough water flow

A tankless heater only lights when enough water flows through it. P1 tells you water is moving, but not enough to reach that minimum, so the water stays cold.

The meaning and service steps below come from Rheem's tankless gas troubleshooting manual, which covers both Rheem and Ruud tankless water heaters. Rheem technical support: (800) 432-8373.

What to check, in order

1. Reset. Rheem's first step: turn the water supply to the unit off, turn the remote off for 10 seconds and back on, restore the water and try a hot tap.
2. Crossed lines. On a new installation, confirm the hot and cold lines are connected to the right fittings on the heater.
3. In-line water filter. Remove, inspect and clean the filter at the cold water inlet. A clogged filter is one of the most common causes.
4. Plumbing crossover. Rheem's test: close the hot water valve at the heater, then open only the hot side at each fixture. If water still flows, cold water is crossing into the hot lines, often through a faulty mixing valve.
5. Low flow at one fixture. If the heater fires when several taps are open, one fixture simply does not draw enough. Clean its aerator screen.

Before you call

Have the model and serial number from the rating label ready, along with the exact code and when it appears: at startup, during use, or only at high demand. Mention any recent work on the gas, water or electrical supply and when the heater was last serviced. This helps the technician bring the right parts and often saves a second visit.

See every code on our Rheem error codes page.