P0108 on 2004-2011 Mazda RX-8: Barometric Sensor Circuit High Causes and Fixes

What's Unique About the 2004-2011 Mazda RX-8

The Mazda RX-8's Renesis engine does not use a traditional Manifold Absolute Pressure (MAP) sensor for engine load calculation; it uses a Mass Airflow (MAF) sensor for that. Instead, it has a separate Barometric Pressure (BARO) sensor to measure atmospheric pressure for fuel and ignition timing adjustments. This sensor is not connected to engine vacuum and is open to the atmosphere. Therefore, unlike on many other vehicles, P0108 on an RX-8 is not caused by vacuum leaks. The fault is almost certainly electrical: a bad sensor, faulty wiring, or a problem with the PCM.

Symptoms You May Notice

• Check Engine Light is on
• Rough or unstable idle
• Poor or jerky throttle response, especially at low RPMs
• Hesitation during acceleration
• Increased fuel consumption
• Black smoke from the exhaust
• Difficulty starting the engine in some cases
• Engine may stall intermittently

Most Likely Causes

1. Failed Barometric Pressure (BARO) Sensor 🔴 High Probability The sensor is exposed to significant heat cycles in the engine bay, and internal electronics can fail over time, causing an internal short that sends a constant high voltage signal (5V) to the PCM.
   How to confirm: With the ignition on and engine off, use an OBD-II scanner to check live data for the BARO sensor. A reading stuck at or near 5 volts, or a pressure reading that is clearly incorrect for your altitude (e.g., 14.7 PSI is sea level), confirms a sensor or circuit issue. You can also back-probe the signal wire at the sensor connector with a multimeter; if it reads 5V, the sensor has likely failed.
   Typical fix: Replace the Barometric Pressure sensor.
   Est. part cost: $30-$220
2. Wiring or Connector Issue 🟡 Medium Probability Engine bay heat and vibrations can cause wiring insulation to become brittle or connectors to become loose or corroded over time. The connector for the BARO sensor can sometimes become contaminated with oil if there are nearby spills during oil changes.
   How to confirm: Visually inspect the wiring harness and connector for the BARO sensor. Look for any signs of damage, corrosion, oil contamination, or loose pins. With the sensor unplugged and ignition on, check for 5V reference and a good ground at the connector using a multimeter. Check for a short to power on the signal wire by testing for continuity between the signal pin and the 5V reference pin.
   Typical fix: Repair the damaged section of wiring or clean/replace the connector pigtail. Apply dielectric grease to the new connector to prevent moisture intrusion.
   Est. part cost: $5-$25

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): → Shop Engine Control Module (ECM) This is extremely rare. The PCM should only be considered a possibility after the sensor and its entire circuit have been thoroughly tested and confirmed to be in perfect working order. A PCM failure would likely be accompanied by other seemingly unrelated error codes.

Diagnosis Steps

1. Read the code: Use an OBD-II scanner to confirm that P0108 is the only code present. If other codes exist, address them first as they may be related.
2. Check Live Data: Using the scanner, view the live data for the Barometric Pressure (BARO) sensor. With the key on and engine off, the reading should be close to your local atmospheric pressure (e.g., ~29.92 inHg or ~14.7 PSI at sea level). A reading that is maxed out (often showing as 5V) is a strong indicator of the fault.
3. Locate the Sensor: The BARO sensor is located in the engine bay, mounted on a bracket near the oil filler neck and secondary air injection pump. It is a small black plastic sensor with a 3-pin electrical connector.
4. Inspect the Connector and Wiring: Disconnect the sensor and carefully inspect the connector pins and wiring harness for any signs of corrosion, damage, oil contamination, or loose connections.
5. Test the Circuit: With the connector unplugged and ignition on, use a multimeter to test the connector pins. You should find a 5-volt reference, a ground, and a signal wire. The wire colors are typically Black/Blue (5V reference), Black/Green (Ground), and Black/Red (Signal). Verify the 5V and ground are present.
6. Test the Sensor: If the circuit wiring tests good, the sensor itself is the most likely culprit. Replacing it is the most straightforward and common fix.
7. Clear the Code and Test Drive: After replacing the sensor, clear the trouble code with your scanner. Start the engine and check the live data again to ensure the new sensor is reading correctly. Take the vehicle for a test drive to confirm the Check Engine Light does not return.

Parts You'll Likely Need

• Barometric Pressure Sensor (OEM #KL47-18-211A) — This is the sensor that provides the atmospheric pressure reading to the PCM. Internal failure is the most common cause of the P0108 code on an RX-8.
  Trusted brands: Mazda (OEM), Denso, Mitsubishi, Standard Motor Products (SMP)
  OEM price range: $180-$220
  Aftermarket price range: $30-$90

Platform-Specific Known Issues

• Sensor Location: The sensor is easily accessible, located on a bracket near the oil filler neck. It can be replaced in minutes with a 10mm socket or wrench.
• Owner Experience: A user on Reddit with a 2006 RX-8 reported the P0108 code with no other symptoms. The community quickly identified the BARO sensor next to the oil fill cap as the culprit, noting it could be a bad sensor or have oil in the harness.

Mechanic-Grade Diagnostic Values

• BARO Sensor Signal Voltage (Key On, Engine Off) — expected: Approximately 2.75V at sea level, decreasing with altitude. Should match a known-good sensor or specification for your altitude.. Failure: A reading stuck at or near 5.0V indicates an internal short or a short-to-power in the signal wire. A reading near 0V would trigger a P0107 (Circuit Low) code.
• BARO Sensor Voltage vs. Pressure — expected: The sensor outputs a linear voltage. Based on a chart for this sensor type, ~2.75V corresponds to ~14.7 PSI (sea level), while 4.0V corresponds to roughly 14 PSI. The P0108 triggers when voltage exceeds the expected maximum, often sticking at 5V.. Failure: Any voltage reading that does not correlate with the local atmospheric pressure or is fixed at 5V.
• PCM Connector Voltage Check (Sensor Unplugged, Key On) — expected: Pin 4K (Black/Blue wire): ~5V Reference. Pin 4U (Black/Green wire): ~0V (Ground). Pin 1S (Black/Red wire): Should show 0V (this is the signal wire).. Failure: If Pin 1S shows 5V with the sensor unplugged, there is a short-to-power in the wiring harness between the PCM and the sensor connector.

Wiring & Ground Locations

• BARO Sensor PCM Pins — At the main Powertrain Control Module (PCM) connector.. Allows for testing the circuit directly at the PCM to isolate a wiring fault from a sensor fault. The pins are: Pin 4K (5V Supply), Pin 4U (Sensor Ground), and Pin 1S (Signal Input).
• Engine Harness Grounds — There are several key grounds. One is on the lower intake manifold. Two are on the crossmember under the battery/airbox. Two are on the passenger side harness to the chassis (near strut tower and washer bottle). One is on the driver's side harness to the chassis near the brake booster.. A poor ground connection on the sensor circuit (Pin 4U) can cause floating voltages and incorrect readings, potentially triggering a high or low circuit code. Checking the main harness grounds is a crucial step if the sensor and wiring test good.

Real Owner Repair Stories

• RX8Club.com user 'luke0b' (Mazda RX-8 (year not specified, pre-2013)) — Engine over-fueling, backfiring on throttle lift, failed emissions test (high ppm).
  ❌ Tried (didn't work) Replacing catalytic converter, Replacing O2 sensors, Replacing coils and plugs, Changing injectors
  ✅ What actually fixed it The user identified the BARO sensor reading was stuck at 5V. While the thread doesn't have a final confirmation post, the clear identification of the stuck 5V reading after replacing all other components points directly to a failed BARO sensor as the root cause, which is the standard fix for this symptom.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Premature Engine Wear / Low Compression 🔴 High — Very common, especially on engines over 60,000-80,000 miles. Caused by apex seal, side seal, and housing wear. Symptoms include difficulty starting when hot, rough idle, and power loss.
• Ignition System Failure (Coils, Plugs, Wires) 🔴 High — Ignition coils have a high failure rate and are considered a regular maintenance item, often replaced every 30,000 miles. Weak coils lead to poor combustion, which accelerates carbon buildup and engine wear. (Ref: TSB 01-016/07 provides a supplementary inspection procedure for ignition coils.)
• Engine Flooding 🟠 Medium — Common if the engine is shut off before reaching full operating temperature. Excess fuel washes oil from the housing walls, making a restart difficult. Mazda issued a TSB with a PCM reflash and a 'de-choke' procedure to address this. (Ref: TSB 01-011/04 addresses the crank/no-start condition due to flooding.)
• Catalytic Converter Clogging/Failure 🟠 Medium — The rotary engine's high exhaust gas temperatures and potential for oil consumption can cause the catalytic converter to overheat, melt, and clog. This creates excessive backpressure, which can severely damage the engine.
• High Oil Consumption 🟡 Low — By design, the Renesis engine injects oil into the combustion chambers to lubricate the apex seals. This is normal operation, but requires owners to check and top off oil levels frequently (e.g., every other fuel fill-up). Failure to do so leads to rapid engine wear.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used OEM sensor from a low-mileage donor car can be a cost-effective option if the price is significantly lower than a new aftermarket part. Since failure is electronic and heat-cycle related, 'used' is a gamble, but can be viable for a quick, cheap fix.

Donor-vehicle mileage cap: roughly under 60000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Ensure the part number (KL47-18-211A) matches exactly.
• Inspect the plastic housing for cracks or signs of extreme heat stress.
• Check the electrical connector pins for any corrosion or damage.
• Ask the seller if the part was tested or if the donor car had any running issues or similar codes.

Aftermarket brands forum-validated for this vehicle:

• Denso
• Standard Motor Products (SMP)
• Mitsubishi (who may be an original supplier)

Brands owners have reported issues with on this vehicle:

• Unnamed, no-brand sensors from online marketplaces can have poor quality control, leading to inaccurate readings or premature failure. It is better to spend slightly more on a reputable aftermarket brand.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0107 — This is the 'Circuit Low' counterpart to P0108, involving the same Barometric Pressure (BARO) sensor circuit.
• P0171 — A lean condition code that may appear alongside P0108 if the BARO sensor fault causes the PCM to incorrectly calculate the air-fuel mixture, leading to increased fuel consumption or black smoke.
• P0300 — Random misfire code; while P0108 relates to the BARO sensor, the RX-8 is highly prone to ignition system failure (coils, plugs, wires) which can cause similar symptoms like rough idle and hesitation.
• P0420 — Catalytic converter efficiency code; related because the over-fueling caused by a faulty BARO sensor (sending a constant 5V signal) can lead to high exhaust temperatures that melt or clog the converter.

Frequently Asked Questions