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P0508 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Definition

When the idle air control system circuit is low, this code appears and is stored in the onboard computer.

Meaning

When the engine idles too slowly, the P0508 code is displayed. The powertrain control module has detected an inconsistency in the engine's RPMs. When the PCM detects that the engine is idling too fast or slowly, it will attempt to correct it. If unsuccessful, it will save the code.

Causes

The following are some of the most common reasons why this code may fire:

Common Causes

  • Faulty (IAC) Air Idle Control Motor Assembly
  • (IAC) Air Idle Control Motor Assembly motor harness could be shorted or open
  • Vacuum Leaks at Intake Manifold
  • Damaged or Corroded Throttle Body Plates or Actuator Motor
  • Bad PCM Software or Hardware Failure

Symptoms

The P0508 code will produce a Check Engine Light on the instrument panel/dash and will likely affect:

Rough Idle – When the engine is running, there are significant RPM variations that are not normal

Engine Hesitates or Stumbles – When the engine is running, it may hesitate or stumble upon acceleration, which can cause poor performance and delayed acceleration

Surge While Driving – Engine surges when under load (as in during acceleration)

Loss of Power at Higher Speeds – The vehicle lacks power while driving on highways/freeways and performs poorly at high speeds.

Diagnosis

P0508 diagnosis can be tricky, as this code could mean something other than an idle air control motor/assembly problem or there may be multiple reasons for the same issue. A qualified technician can read the data from the scan tool to determine when the problem occurred, or if it is still occurring. They may clear the code/light and test drive the vehicle while monitoring data to see if it returns.

The Idle Air Control Motor (IAC) is a small electric motor that controls the engine's idling speed by regulating airflow into the engine. It has a very simple lowering device to control airflow- either raising or lowering its armature according to RPMs sensed by the PCM. The IAC moves up and down very quickly during warm-up, stopping for about 30 seconds at all other times. It accomplishes this through the use of internal springs which open and close valves to air intake.

Common mistakes

P0508 is a specific code. It is not generic like most other trouble codes, and differentiating it between similar codes can be difficult. The two most common mistakes are the IAC motor itself, and low voltage at the PCM.

The IAC Motor Assembly can go bad or become clogged with carbon deposits over time, causing it to function poorly or stop working altogether. Because of this, it is commonly misdiagnosed as P0508 instead of being addressed as a separate issue to be resolved first before attempting additional repairs/testing on things that could possibly cause other issues.

In some cases, especially those involving older vehicles, the wiring from either the IAC motor assembly or from the PCM to its sensor may have been cut by rodents or melting due to road debris/traffic incidents through insulation and into wires. In these cases, corrosion builds up in exposed wiring, or cuts are left open, which can cause extremely low voltage during startup. This oftentimes will look like the IAC motor itself is malfunctioning.

How serious is this?

P0508 is one that you do not want to ignore. If the idle RPMs are too high or too low, it can cause extensive damage to other parts of your engine and it will prevent the vehicle from running correctly. 

If the vehicle hesitates, stumbles, or has a loss of power during acceleration, it can also cause extensive damage to other parts in your engine and it may prevent the vehicle from running correctly. If the vehicle surges while driving, it can be very hazardous while operating and should be resolved as soon as possible.

What repairs can fix the code?

The most common potential repairs to address the P0508 code are as follows:

First, they'll check to see whether the P0508 code was triggered by accident. If it's accurate, they'll clean and reset the throttle body bore before rechecking the code. They'll then inspect the electrical connections and circuits of the IAC for good functioning, making any necessary repairs, and replacing components as needed. In certain cases, they may need to replace the IAC. They may also need to replace the PCM; however, this is extremely uncommon.

Related codes

The P0508 code could be accompanied by other trouble codes, including: 

P0171, P0174, and others that indicate a vacuum/air pressure issue.

P0507, P0508, P0521 and others that indicate an electronic throttle control issue.

P1635 and other codes that indicate an issue with the battery/charging system.

How much does it cost to fix the P0508 code?

Repair cost will mostly rely on the type of repair performed and how extensive it was; however, we can determine a price range for parts and labor.

The IAC may only require cleaning and resetting, which typically costs around $75-100. However, if the problem is deeper than that or other issues need to be addressed (like wiring repairs), then pricing will rise into the $300's/400's for both labor and parts.

Replace the PCM: $200-$700 for labor + $150-$900+ for part(s). If replacing certain components with the PCM such as relays, sensors or other modules add another $50-$150 each to replace those at the same time.

Finally, if wiring or other issues are found during diagnosis, expect another $100 to help cover those expenses.

Conclusion

P0508 is one that you do not want to ignore. If the idle RPMs are too high or too low, it can cause extensive damage to other parts of your engine and it will prevent the vehicle from running correctly. 

If the vehicle hesitates, stumbles, or has a loss of power during acceleration, it can also cause extensive damage to other parts in your engine and it may prevent the vehicle from running correctly. If the vehicle surges while driving, it can be very hazardous while operating and should be resolved as soon as possible.

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