1998 Ford Explorer P0420 and P0174 Codes: Cold Start Issues, Misfire, and Potential MAF/Vacuum Leak Diagnosis

Do you have access to an O2 sensor scanner? If yes, please share live data—this helps rule out faulty post-cat oxygen sensors or catalyst issues before replacing components. Before blaming the catalytic converter or post-catalyst O2 sensor, try disconnecting the MAF sensor and see if idle stability improves. If it does, cleaning or replacement of the MAF may resolve the issue. Additionally, check fuel pressure and inspect all intake hoses and vacuum lines for leaks—especially when the engine is cold. Was a recent tune-up performed? Bank 1 refers to the side with cylinder 1. Good luck and please update us on your findings.

Vacuum leak is likely the root cause of both symptoms—especially rough idle and lean conditions in cold weather.

A dirty MAF sensor would typically affect both banks equally, causing a consistent lean condition. Prolonged lean operation can damage the catalytic converter, leading to P0420. However, a post-catalyst O2 sensor failure is not typically the cause of P0420; it usually results in a different code or misreads under specific conditions.

I removed the MAF and the engine ran briefly before dying—no sputtering, just sudden shutdown. I cleaned the MAF, air filter, intake canister, and throttle body flange. The symptoms remain unchanged. The clicking noise is coming from a pulley under the power steering fluid reservoir—it clicks as it spins and stops repeatedly. At idle, RPMs sometimes spike to 2000+ before dropping again. I don’t have O2 sensor monitoring tools. This is very detailed—thank you for your patience! Any suggestions?

Rough idling in cold weather strongly points to a vacuum leak. Are there any visible leaks or loose hoses on the intake system?

I agree with Jeff—vacuum leak is likely. Have you checked for leaks? The clicking sound may be caused by the air conditioning compressor clutch engaging and disengaging. If your A/C or defrost is on, try turning it off—the noise should disappear. This could help confirm whether the issue is mechanical or related to climate control systems.

I have identical P0420 and P0174 codes on my 2000 Toyota Corolla with 92,800 miles. After a long drive (800 miles), the P0420 appeared, followed by P0174. I cleaned the MAF sensor—it was fairly dirty—but still didn’t fix it. Took it to Firestone; they suspect a vacuum leak and charge $261 for a full inspection (3 hours spent). They claim they can hear it but not visually locate it. Should I proceed with that cost? Am I on the right track?

MAF issues are possible, but only if tested properly. Avoid spending money without a trained diagnostic technician—especially one experienced in drivability and emissions systems. Tire shops often lack the proper tools or expertise for accurate diagnosis.

Oops—I made a mistake in my original post. The actual codes are P0420 and P0171, not P0174. I had an ASE-certified technician perform diagnostics. I don’t have any other nearby shops to choose from—so I’m considering going ahead with the $261 vacuum leak inspection. Am I making a reasonable decision?

ASE certification does not guarantee diagnostic competence. Some certified technicians lack real-world experience or technical judgment. Diagnostics require more than just credentials—experience, knowledge application, and customer-focused problem-solving are essential. Many ASE-certified techs at tire shops may not be qualified for complex engine issues.

I agree with COMP^. ASE certification is a baseline but doesn’t ensure quality diagnostics. I’ve met several certified technicians who lack practical skills or diagnostic insight. True expertise comes from applying knowledge to real-world scenarios and offering thoughtful, accurate solutions.

It’s possible the technician at the shop is not actually wearing their own shirt—many ASE-certified staff rotate frequently. With high turnover in service shops, this could be more common than people realize!

Exactly—it feels less like a professional repair facility and more like a retail store (like Walmart or Costco).

Agreed! I was quite helpless—no nearby alternatives and no spare vehicle. Still, I’ll keep updating on progress. Thanks for the input!

The diagnostic team concluded that the MAF sensor needs replacement, charging $328. Toyota dealerships sell a similar part for $168. Should I replace it myself at $168? What do you recommend?

I’ve read your post—no, the MAF is unlikely to cause lean conditions on just one bank. A single-bank issue suggests a localized problem like a vacuum leak or intake gasket failure. Without live O2 sensor data or fuel pressure readings, I wouldn’t recommend replacing the MAF. I also don’t trust Firestone’s diagnosis—they haven’t provided clear reasoning for their conclusion. Avoid spending $168 on a replacement without further evidence. Focus first on identifying vacuum leaks or intake system issues.

I agree—$168 for a MAF sensor would be a waste if cleaning didn’t help. If the MAF isn't the issue, it’s unlikely to be the root cause. Has anyone checked the O2 sensor readings? Is the system consistently running lean? Can you drive the O2 signal into a rich condition? Has fuel pressure been tested?