Error Code 12
Low

Rheem R96VA0702317MSA Error Code 12: Low Flame Sense

TL;DR
Code 12 means the flame sensor is detecting a weak signal — the furnace is still running, but the sensor is marginal and may soon fail to detect the flame at all, leading to a more serious fault.
Disclaimer
This article is for informational purposes only and does not guarantee the accuracy or completeness of the information provided. Always turn off power and gas supply before attempting any repairs. If you smell gas, leave immediately and call your gas company. Consult a licensed HVAC technician for diagnosis and repair. Any actions taken based on this information are at your own risk.

What Does Code 12 Mean?

Fault code 12 on the Rheem R96VA0702317MSA indicates that the IFC (Integrated Furnace Control) is receiving a low or marginal flame sense current from the flame sensor rod. Unlike most fault codes, this one is non-critical: the furnace continues to operate normally in heating mode. The IFC records the condition and keeps running, so you may not notice any change in heating performance.

The concern is what happens next. The flame sensor works by passing a small electrical current through the burner flame, and a contaminated or corroded sensor rod produces a weaker signal over time. If the signal weakens further, the IFC will no longer be able to confirm combustion at all — escalating to code 13 (flame lost after ignition) or code 11 (ignition failure with lockout). In short, code 12 is a warning that a service issue is developing before it becomes a heating outage.

The most common cause is an oxide or carbon buildup on the stainless steel sensor rod. This buildup is normal over time and develops faster in dusty environments or with dirty filters that allow more particulates into the combustion air. A less common cause is a damaged or cracked porcelain insulator on the sensor, which can cause current leakage and a weak signal. Call a technician if cleaning does not resolve the fault, if the porcelain insulator is visibly cracked, or if code 12 returns within a single heating season after a cleaning.

Common Causes

Cause Likelihood DIY?
Dirty flame sense rod needing cleaning Most common ✓ DIY fix →
Flame sensor wiring shorted or improperly connected Common ✗ Call a pro →
Improperly mounted or poorly grounded flame sensor Uncommon ✗ Call a pro →

How to Fix It: Clean the Flame Sensor Rod

⚠ Safety First
Always turn off the furnace at the power switch or breaker and shut off the gas supply before beginning. Do not proceed if you smell gas — leave the area and call your gas company immediately.

What You'll Need

Steps

  1. Turn off electrical power AND shut off the gas supply valve Flip the furnace circuit breaker to OFF (or use the power switch on or near the furnace). Turn the manual gas shutoff valve to the OFF position (handle perpendicular to the pipe). If you smell gas at any point during this procedure, leave the house immediately and call your gas company from outside.
  2. Remove the furnace access panel and locate the flame sensor Open the burner compartment access panel. The flame sensor is a thin metal rod — usually slightly bent — with a porcelain insulator at its base. It is positioned near the burner assembly so the rod sits in the flame path during operation. A single wire connects to it and it is held by one 1/4-inch screw.
  3. Disconnect and remove the sensor Pull the wire connector off the sensor terminal. Remove the single mounting screw with your 1/4-inch nut driver and carefully slide the sensor out. Hold it by the porcelain insulator or mounting bracket — touching the rod with bare fingers can leave oils that contribute to future buildup.
  4. Clean the sensor rod Gently rub the metal rod with a Scotch-Brite pad, working along the length of the rod until the surface is clean and bright. Do not use steel wool, sandpaper, or emery cloth. Clean only the metal rod — avoid abrading or damaging the porcelain insulator. If the insulator is cracked, replace the sensor rather than reinstalling it.
  5. Reinstall the sensor Slide the sensor back into its mounting bracket, start the screw by hand to avoid cross-threading, and tighten snugly. Reconnect the wire. Confirm the rod is positioned to sit within the burner flame when the furnace fires.
  6. Restore power and gas, then test Turn the gas shutoff valve to the ON position (handle parallel to the pipe) and restore electrical power at the breaker or switch. Set your thermostat above room temperature to call for heat and let the furnace complete a full heating cycle.
How to Verify
Let the furnace run through at least one complete heating cycle — from ignition through the full heat call until the thermostat is satisfied. Listen for the burners to light and stay lit steadily. If the furnace completes the cycle without displaying fault codes, the cleaning was successful. If code 12 or code 13 appears again after a cleaning, the sensor likely needs replacement.

When to Call a Professional

Contact a licensed HVAC technician if:

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Sources

  1. Installation Instructions for 4 Position Condensing Two-Stage, Communicating Gas Furnaces w/ECM Blower (-)96V Series & (-)(-)96MDV Series

✓ Verified against manufacturer service manual — March 2026