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2002 Mitsubishi Lancer OZ Rally - P0421 Code Fix & Missing Vacuum Control Valve (California Smog Ready)

Model: 2002 Mitsubishi Lancer OZ Rally Fault Code: P0421 Posted: 2015-03-03 10:50

I recently acquired a 2002 Mitsubishi Lancer OZ Rally with 240,000 miles. After fixing several previous issues—P0401, P0507, P0441, and a misfire caused by oily spark plugs—I began seeing a pending P0421 code. The spark plugs and wires were replaced during this process. I've thoroughly cleaned the throttle body, IAC valve, EGR, and Mass Air Flow (MAF) sensor. Recently, I installed a new valve gasket and discovered that two vacuum hoses had been incorrectly swapped: one connecting the purge valve to the canister was mistakenly routed to the PCV breather line. Additionally, I found two restrictions in the vacuum lines—first, a small check valve, and second, a yellow electrical tube with cotton inside the EGR purge line. I replaced all affected vacuum hoses, cleaned all ground connections and battery terminals, and ensured proper electrical continuity. After these repairs, the engine runs less smoothly than before, and the P0421 code has transitioned from pending to active after approximately two hours of driving with five stops. According to my underhood VEC (Vehicle Electronic Control) system, there should be a vacuum control valve (VCV), but it is missing. This is critical as I need to pass California smog inspections. I am seeking advice on how to resolve this issue without costly mistakes—especially since the P0421 code relates to catalytic converter efficiency and oxygen sensor performance, which may be affected by vacuum system integrity or sensor calibration.

Related fault codes
P0421
Comments (7)
Anonymous 2015-03-03 12:17

Use an OBD-II scanner to monitor pre-cat and post-cat oxygen sensor readings. Refer to https://www.obd-codes.com/p0421 for detailed troubleshooting tips. With only the P0421 code present, check if all emission monitors are set to 'READY' or 'OK'. I can't confirm California's smog testing requirements from afar, but typically, a vehicle must have all monitors in READY status with no active fault codes and a functional CEL (Check Engine Light) system.

Anonymous 2015-03-03 13:02

The EVAP and catalytic converter monitors were inconclusive. The engine is running poorly—experiencing high idle in park and rough idling in drive. I accidentally left the MAF sensor connector disconnected when starting the engine. Could this have caused any damage? The engine sounds unusually noisy, possibly due to a belt issue. A previous mechanic ignored the P0421 code and simply raised the idle speed to compensate—could this have affected timing or long-term performance?

Anonymous 2015-03-03 13:15

This is a used vehicle acquired just 14 years ago—many components may be outdated or improperly serviced. Unplugging the MAF sensor does not cause harm in most cases and can help test its circuit function. Vacuum leaks are likely contributing to high idle. Focus on inspecting EGR and purge vacuum lines as these are common sources of issues. Is this a 2.0L non-turbo engine? Consider searching for official vacuum diagrams specific to the 2002 Mitsubishi Lancer OZ Rally, and verify whether it meets federal or California emissions standards.

Anonymous 2015-03-03 13:17

I just rechecked the system—all eight emission monitors are now in 'READY' status, including the catalytic converter monitor. This suggests that the issue is not likely a failing catalytic converter (though it's still possible).

Anonymous 2015-03-03 13:30

The current issue is solely P0421—no other codes are present. It’s recommended to use an OBD-II scanner to monitor oxygen sensor voltage readings. The link I provided for P0421 includes key diagnostic considerations—please review it before proceeding with any repairs.

Anonymous 2015-03-03 13:35

By the way, thanks for your quick response! In my last post, I mentioned multiple codes—I’ve since resolved those. How many oxygen sensors does this vehicle have? Is it one pre-cat and one post-cat?

Anonymous 2015-03-03 13:53

There are two oxygen sensors: one before the catalytic converter (pre-cat) and one after (post-cat). The issue lies with the post-cat sensor—its performance is directly affected by the relationship between the pre-cat sensor and the vacuum system. Using a scanner to monitor actual voltage readings is the most professional approach. Replacing sensors based on intuition may not be cost-effective—sometimes diagnostic time (e.g., 30 minutes at $100/hour) outweighs replacing a $50 part. Avoid cheap or DIY sensor replacements—always use OEM-quality parts and ensure proper installation.