What is CID 991 & How to Solve It?

What is CID 991:

cid 991 refers to a problem in the exhaust aftertreatment temperature sensor circuit. this sensor helps manage the diesel particulate filter (dpf) regeneration process, and failure can lead to high soot buildup or warning lights.

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Why This Happens:

• failed aftertreatment temperature sensor.
  
• melted or damaged wiring due to heat.
  
• blocked exhaust passages.
  
• corroded or loose connectors.
  
• ecm failing to process temperature readings.
  
  

How to Fix It:

1. inspect sensor wiring for heat damage.
   
2. test resistance at ambient and hot exhaust temperatures.
   
3. clean exhaust piping and remove soot buildup.
   
4. replace faulty sensor if readings remain off.
   
5. confirm proper dpf regeneration after repair.
   
   

How to Avoid It Before It Happens:

• maintain clean exhaust and dpf systems.
  
• inspect wiring for heat insulation integrity.
  
• avoid extended idling that causes soot accumulation.
  
• clue’s fleet management software monitors exhaust aftertreatment systems to prevent regeneration failures and downtime.