Tankless: start first
How every tankless heater works
1) A hot tap opens and water flows in. 2) A flow sensor (a small spinning wheel) tells the board water is moving. 3) The board starts the blower. 4) An air pressure switch proves air is moving. 5) The igniter sparks. 6) The gas valve opens. 7) The flame sensor proves there is a flame. 8) The outlet thermistor reads the hot water temperature and the board trims the gas to hold the set temperature. 9) Tap closes, it shuts down.
If ANY step fails the board stops and shows a code. The code tells you WHICH STEP failed. It does not always tell you which part is bad.
Why they are hard to diagnose
The whole start sequence takes seconds. A failed try often locks out after 2 tries, so you have little time to watch. A tankless needs a minimum water flow to even start (often about 0.5 to 0.7 GPM), steady 12V, and full LP pressure while the burner is at high fire. Weak anything looks like a bad part.
Most tankless calls are NOT a bad heater. They are low flow, low 12V, low LP pressure, reversed hot and cold, or a blocked vent.
6 checks BEFORE you read a code
12V at the heater board, measured WHILE it tries to start (above about 11 V)
LP tank has gas and the valve is fully open
Hot and cold lines are not swapped at the heater
Water flow is enough: fill a gallon jug from the hot tap (see Flow test below)
Inlet screen, tap aerators and any water filter are clean
Exhaust and air intake are clear (no wasp nests, no covers, no wind)
Where did it stop? Watch and listen
Nothing happens at all, no blower: not enough flow, no 12V, tripped ECO or bad board.
Blower runs, no spark: air pressure switch or its hose, blocked vent, bad board.
Spark, no flame: LP supply or pressure, gas valve not opening, igniter gap, air in the gas line after a tank change.
Flame lights, then goes out in a few seconds: flame sensor, poor ground, low gas pressure at high fire, vent recirculating exhaust.
Runs but water is lukewarm: flow too high for the burner, low gas pressure, scale, bad outlet thermistor.
Hot, cold, hot, cold: low flow, a mixing faucet or outside shower, an old flow converter, a failing inlet or outlet thermistor.
Flow test (do this on every no-start)
Open the hot tap fully at the farthest sink. Time how long a one gallon jug takes to fill. About 1 minute 25 seconds or less is enough flow for the Girard (that is about 0.7 GPM). Slower: find the restriction.
Restrictions to check: pump or accumulator, inlet filter screen, faucet aerator, kinked PEX, a half-closed bypass valve, a 'water saver' flow restrictor in a shower head, low water pressure at the campground.
Reversed hot and cold at the heater is a classic. The flow wheel will not spin. It looks like a dead heater.
Water side
Bypass valves: confirm they are set to normal, not bypass. Outside shower and single-handle faucets can mix cold water into the hot line and make the heater swing. Test at a simple two-handle faucet. Check for a failed check valve letting cold water cross over.
Hard water scale builds up inside the heat exchanger and loosens the flow wheel. Flush with a mild descale solution per the maker. Many models have a dedicated descale routine (Truma has one built in).
Gas side
Girard and Furrion units list about 11 to 14 in W.C. at the inlet while running. Read it with a manometer ON a running burner, not just static. A pressure that is fine at rest but drops at high fire means a bad regulator, too-small hose, a long line, or a nearly empty tank.
After a tank change the lines can hold air. Run a stove burner until it lights, then retry the heater.
12V side
Measure at the heater, not at the battery. Run the blower and the gas valve together and watch the meter. Under about 10 V the board resets or throws odd codes. Voltage drop on a thin or corroded wire is the usual cause.
Rules
Never bypass the ECO (high-limit) switch. It is a safety. If it is open, find out WHY.
Soap-test every gas fitting you touch. Test again after the repair.
Write the code down BEFORE you reset anything. Resetting clears the clue.
Next: open the topic for the brand. If the code chart is not here, use the maker's chart on the lid or the manual.