Carrier's check-for list for code 34, in the manual's own order of emphasis: oxide buildup on the flame sensor (cleaned with fine steel wool — by the technician, inside the burner compartment), proper flame-sense microamps (0.5 minimum, 4.0-6.0 nominal), a manual gas valve left shut, low inlet gas pressure, control ground continuity, a defective hot-surface ignitor, a defective or turned-off gas valve, poor flame carryover.
Your slice of that list is the gas valve check (is the shut-off at the furnace open?), the thermostat call, and one reset. The rest is meter work. Note the escalation: the control retries three more times, then gives up into code 14, ignition lockout, with a three-hour clock.
Homeowner checks — the safe list:
- Thermostat set to Heat, and set a few degrees above the room temperature.
- The gas shut-off valve at the furnace: handle in line with the pipe means open. If it is off, that is your answer — and ask yourself why it was off.
- Air filter: pull it and look. A matted filter causes more furnace faults than any broken part.
- One reset is fair: power off at the switch for 30 seconds, back on, and watch what it does. If it locks out again, stop resetting it — the code is telling you something real.
Code 34: Ignition proving failure
Homeowner checks
The furnace lit (or tried to) and could not prove flame; it retries three more times before lockout 14. Manual check-for: oxide buildup on the flame sensor, a shut manual gas valve, low inlet gas pressure, a dead hot-surface ignitor, a defective gas valve, ground continuity.
From the table: Two-digit status codes — short flashes, then long flashes — source: Carrier 58YAV Service section - status code table (read the document, fetched 2026-10-30). Covers: Carrier 58-series and their Bryant/Payne twins. First digit = number of short flashes; second digit = number of long flashes. Code 33 is 3 short, pause, 3 long.
Why the flame sensor fails politely
The sensor does not break; it coats. Combustion leaves an oxide film on the rod, the film insulates it, and the flame signal sags below the half-microamp floor the board demands. It is the most common no-heat call in the Carrier world and the cheapest part on the truck — which is why a 34 on an older furnace is usually a cleaning, not a catastrophe. It is also, on this site, firmly a technician task: the sensor lives in the burner compartment, and our rules stop at the cabinet door.
The two rules that outrank everything on this page.
1. Smell gas? Stop reading. Leave the house and call your gas utility or 911 from outside. No code is worth diagnosing through a gas smell.
2. Never bypass a safety switch — no jumping a pressure switch, no taping a limit, no propping a rollout. Those switches are the only thing between a furnace fault and a fire or carbon-monoxide problem. This library explains codes; it does not teach repairs inside the cabinet.
One honesty note before the tables: codes change between control boards, sometimes inside the same brand and the same decade. Every table below names the board and the manual it was transcribed from. The chart glued inside your own blower door is the final word for your furnace — if it disagrees with this page, believe the door.