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Mazda Miata P0661: Meaning, Causes and Diagnosis


P0661 Mazda Miata

P0661 is a relatively rare OBD2 fault code. Indicates that the electrical reading coming from your Mazda Miata’s intake manifold tuning valve control circuit (on the bank 1 side) is below its specified operating range.

While P0661 is a generic code, it has the same definition for all vehicles.

P0661 Definition: Intake Manifold Trim Valve Control Circuit Low (Bank 1)

P0661 Mazda Miata Code

Here is the definition of P0661 for the Mazda Miata, broken down into its three parts.

Intake manifold trim valve check

The intake manifold tuning valve control system is responsible for redirecting air into your Miata’s intake manifold

Relieves air pressure on the opposite side of the intake manifold. Or, you can channel the air through a completely different duct for power/efficiency purposes.

Low circuit

The ECM or PCM (your Miata’s central computer) receives a signal from the intake manifold adjusting valve. There is a minimum and maximum voltage range that the PCM expects to see.

P0661 will be stored in the computer memory when this voltage level is below the minimum level

If there is no voltage, you will get code P0660. That code indicates that the intake manifold adjusting valve circuit is open.

Bank 1

Bank 1 is the part of your vehicle’s engine with the first cylinder. You don’t have to worry about this if you have an inline-four or inline-six engine.

Mazda Miata P0661 Symptoms

Although you may not notice anything wrong when your Miata has P0661, we recommend that you have your vehicle checked as soon as possible due to the location of the intake manifold adjusting valve. If it were to break (however unlikely) the engine could explode.

These are some of the most common causes of P0661

  • Repeated intake noise (probably the intake manifold adjusting valve has failed)
  • Misfires (due to poor air-fuel mixture)
  • Engine underpowered throughout RPM range
  • Reduced fuel consumption
  • Rough idle

Mazda Miata P0661 Causes + Diagnosis

P0661 Diagnostic

Here are the most common causes of P0661, as well as a solid diagnostic order to approach repairing your Miata.

1. Check the manufacturer’s communications

Mazda sends communications through “technical service bulletins”, commonly referred to in the industry as TSBs. Although this site does not maintain a database of them, they are easy to find online

If you click the link above, it will take you to the NHTSA website in a new window. Find your exact make, model year, and engine type. A TSB related to the intake manifold adjusting valve gives you a good starting point for diagnosis with a known failure point. It is imperative that you use the correct model year, as bulletins change from year to year.

2. Inspect the wiring harness

Start by locating the intake manifold and adjusting the valve harness. It will be accessible in the intake manifold. Inspect the wiring harness for damage leading to and from it

Make sure the pins where it connects to the intake manifold adjusting valve are not damaged or corroded in any way. If they are, you will need to repair them, renail them, or replace them as necessary.

Rodent problems sometimes cause this code. They like to lie on a hot engine when it’s cold and can damage the wiring harness, especially around the intake.

3. Check the voltage

Using a voltage gauge, check that the wiring harness leading to your Miata’s intake manifold control circuit has the proper voltage. Check right in the wiring harness that goes to and from the intake manifold adjusting the valve

  • If it has the proper voltage going to it, check to see if the signal going back to the PCM has a voltage level that is within range.
  • If the return voltage signal is too low, it is likely that the intake manifold tuning valve has gone bad.
  • If the flyback voltage IS WITHIN RANGE, you should check it when entering the PCM. If it has lowered, you may have a problem as it travels through the wiring system.

Make sure the mass used for the intake manifold tuning valve is solid. If it is loose or looks like there may be corrosion underneath it, that could easily cause P0661.

4. Order ignition of the intake manifold adjustment valve

Most higher-quality scanners can tell your Miata’s intake manifold adjusting valve to activate. If you hear a clicking noise coming from the intake when it is activated, this indicates that the adjustment valve is stuck or clogged.

Unless you ran the vehicle without an air filter, it’s not very plausible that something was stuck there, but it’s not implausible. Check the air filter for a squirrel nest or other rodent problem. If it looks good, you should be able to rule out the blockage.

The jam remains, which can occur. These adjustment valves are usually plastic and can become stuck and P0661 will be stored in the PCM memory.

5. PCM

It’s rare for a PCM to go bad, but it does happen. If the voltage seemed good going in and out of the intake manifold tuning valve, and the voltage level going to the PCM was correct, that would be enough evidence to consider that the PCM is bad.

Have a professional mechanic take a look at it before purchasing a new PCM to clear the P0661. They can verify that you haven’t missed anything. Additionally, they have diagnostic equipment that can locate voltage drops quickly and easily.


Although there are quite a few reasons why your Mazda Miata may have P0661, the most common are wiring problems or a bad intake manifold adjustment valve. Good luck diagnosing your vehicle!


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