B3491 Subaru Hm Diagnostic Trouble Code

B3491 Subaru Hm code definition:
B3491 is a kind of OBD-II Diagnostic B-Body Diagnostic Trouble Code for Subaru Hm.

B3491 Subaru Hm is a DTC or a diagnostic trouble code. It is a check engine light code which indicates that the catalyst's system efficiency is less than the required threshold. In simple terms, it means that more pollution is added into the air than what was supposed to be because of your car. There are two wires dedicated for each coil; one is battery feed which usually comes from the distribution center, and the other one for the coil driver circuit from the PCM. The PCM grounds or ungrounds the circuit to activate or deactivate the coil.

Symptoms

Symptoms of a B3491 Subaru Hm diagnostic trouble code may include:
Place of B3491 in engine

Possible Causes :

  • Some aftermarket accessories and performance modifications
  • A faulty oxygen sensor. This will set a code in itself, however, a faulty oxygen sensor does not automatically condemn the sensor. The code just means that the sensor signal was not within specifications. An air leak or any of the above will cause an erroneous signal. There is a multitude of O2 codes relating to O2 performance which gives a clue to the problematic area.
  • A large vacuum leak would cause a massive amount of un-metered air to enter the intake manifold resulting in an overly lean mixture.

Common Misdiagnoses

  1. IC Circuit High Voltage Problem
  2. Control Module Throttle Actuator Position Performance Problem
  3. Air Conditioning (A/C) Clutch Feedback Circuit High Voltage Problem
  4. Cruise Vehicle Speed/Set Speed Difference Too High Problem
  5. PCM Stack Overrun Problem

Repair Processes :

If the engine displays a miss and there wasn't a code, determine which cylinder is misfiring. If the exhaust manifold is visible, spay or pour a small amount of water on each cylinder exhaust port. Water will evaporate immediately on good cylinders and slowly on the missing cylinder. If this can't be accomplished pull the plugs and check the condition. With the valve disconnected, the red voltmeter lead should be attached to the IMRC valve power wire and the black voltmeter lead connected to ground. If all testing has passed so far yet you still have the same code, check your scan tool and see if it can open and close the IMRC valve.

Process Tips

Diagnostic trouble code B3491 Subaru Hm is not difficult to diagnose and repair. Thus, it always pays to have a qualified technician to look after your vehicle should this error code arise.

Diagnosing this code requires the engine to be in good running shape, without unmetered air entering the engine. If other codes that relates to misfiring condition, lean running or air entering the engine is present, then you must first fix those error codes before B3491 Subaru Hm.

Fortunately, B3491 Subaru Hm is a fairly common OBD code, meaning that it applies to all makes and models of vehicles made after 1996.

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