LOW URGENCY

Rheem Code 12 - Weak Flame Signal

Code 12

Weak or low flame signal detected by flame sensor.

Brand: Rheem (Also covers Ruud) | Equipment: Gas Furnace

What This Code Means

Code 12 on Rheem (Ruud) means the flame sensor detected flame but the signal is weaker than normal. This often precedes a full ignition lockout if not addressed.

Likely Causes (Ranked)

Most common (~80%)

Dirty flame sensor

What: Carbon or oxide layer weakens signal.

How to check: Pull sensor; check for any coating.

Fix: Clean with fine emery cloth.

Shop parts

Common

Low gas pressure

What: Insufficient gas produces weak flame.

How to check: Tech measures manifold pressure.

Fix: Adjust regulator or call gas utility.

Less common

Cracked or aging igniter

What: Weak ignition produces unstable initial flame.

How to check: Watch glow brightness.

Fix: Replace igniter.

Shop parts

Diagnostic Steps

Pull flame sensor and inspect
Clean with emery cloth
Reinstall and cycle power
Watch flame quality
Call tech if persists

Frequently Asked Questions

Is Rheem code 12 serious?

Not immediately critical, but address soon - usually progresses to code 11 lockout.

How to clean Rheem flame sensor?

Cut power, pull sensor (one screw), wipe metal rod with fine emery cloth, reinstall. 5-minute job.

Same on Ruud?

Yes.

Related Rheem Codes

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Need More Help?

Try our interactive symptom diagnostic tool or browse Rheem parts.

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