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1999 Mazda B4000 2WD - P0174 and P0304 Error Codes: Causes, Diagnosis & Fixes

Model: 1999 mazda b4000 2wd Fault Code: P0174 Posted: 2012-02-05 19:51

Hi everyone, I'm having trouble with my 1999 Mazda B4000 2WD. After scanning the engine, I've received two diagnostic trouble codes: P0174 (System running lean in Bank 2) and P0304 (Cylinder 4 misfire). The truck sometimes runs rough, especially when revved above 1500 RPM while in park—after that, it usually runs smoothly. At idle, the engine performs fine. I've already replaced the spark plugs and ignition wires. I tested each coil individually and confirmed they're firing properly with a strong spark. Given these symptoms, I'm wondering if the issue could be related to the Mass Airflow (MAF) sensor or Oxygen (O2) sensor? I’ve already disconnected the MAF sensor’s electrical connection, but that didn’t resolve the problem. What other components should I check next? Are there common causes for both P0174 and P0304 on this model? Thanks in advance! Kevin

Related fault codes
P0174P0304
Comments (5)
Anonymous 2012-02-06 06:02

Could you confirm the engine size? This helps narrow down potential issues.

Anonymous 2012-02-06 13:00

It's a V6 with a 4.0-liter engine.

Anonymous 2012-02-06 17:29

A vacuum leak is likely the root cause—especially in older 4.0L engines like this one, which are prone to leaks when cold. The engine may not show a strong P0171 code because the fuel trim adaptation on Bank 1 hasn't reached its threshold yet. A vacuum leak on Bank 2 could be causing the lean condition indicated by P0174. To diagnose, check for leaks manually or visit your local mechanic or dealership with the engine cold—many professionals use a smoke test or scan tool to detect such issues accurately.

Anonymous 2012-02-06 18:51

Thanks for the tip! Does using soap and water work effectively for detecting vacuum leaks? I’ve heard about carb spray, but I’m concerned it might be unsafe. WD-40 is also recommended—does that help?

Anonymous 2012-02-07 06:08

On many Ford-style engines (including similar Mazda V6s), the traditional soap-and-water method may not detect leaks reliably due to heat distorting results. The most accurate approach involves using a smoke machine introduced into the intake system or monitoring fuel trim readings via a scan tool—watch for sudden shifts in fuel trim as you apply vacuum pressure. If the traditional method fails, these tools are far more effective. I’ve found soap-based methods often give inconsistent results under real engine heat conditions.