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PO305, PO101, PO420 Fault Diagnosis - 1997 Chevrolet Tahoe | How to Fix Misfire & MAF Issues

Model: 1997 Tahoe Fault Code: P0101 Posted: 2009-09-23 12:22

Hi everyone, I'm Tom and I recently purchased a 1997 Chevrolet Tahoe with persistent check engine light issues. The vehicle is showing three diagnostic trouble codes: PO305 (Cylinder 5 misfire), PO101 (Manifold Absolute Pressure sensor fault), and PO420 (Catalyst system efficiency below threshold). I've already replaced the spark plugs, ignition wires, fuel pump, and fuel filter. The truck originally had a fuel pump failure that I addressed by draining the tank (only 3-5 gallons remained) and installing a new pump and filter. After the replacement, it initially wouldn't start—though adding Seafoam helped it start later, running roughly until I filled up with premium gas. It ran smoothly for about six months before the check engine light returned. Now, when driving between 50-65 mph, there's a noticeable vibration that diminishes below 50 mph. When accelerating through this range, the engine shows little response—indicating potential performance issues. I've checked the U-joints and found no play or looseness. The exhaust doesn't emit a rotten egg smell, and I don’t feel heat from the catalytic converter area, which makes me skeptical of a failing converter. The PO420 code suggests possible catalyst inefficiency, but I'm not certain if it's related to the misfire. I have basic mechanical skills and can use diagnostic tools. My budget is limited—so any cost-effective solutions would be greatly appreciated. I’m especially concerned about the cylinder 5 misfire (PO305) and whether it’s due to ignition components or fuel delivery. Thanks for your help, Tom.

Related fault codes

P0101P0305P0420

Comments (4)

Anonymous 2009-09-23 15:53

Welcome back, Tom! PO101 typically indicates a MAF sensor issue. Have you inspected the wiring and connections to the MAF? If your truck has a K&N air filter, it could be causing carbon buildup on the MAF—leading to inaccurate readings and rough idle. While this might not explain a complete no-start, fuel pressure is critical in 1997 Tahoe Vortec engines. Even with a new fuel pump, issues like clogged nozzles or faulty pressure regulators can result in low pressure or inconsistent delivery. Check those components for possible failure.

Anonymous 2009-09-24 08:46

I had the same issue—was trying to inspect Cylinder 5 spark plug and noticed the boot wasn’t fully seated. Upon inspection, I found the metal connector at the top of the spark plug was bent. After properly reattaching the boot and securing it firmly, the check engine light turned off while driving to my wife’s workplace. No more vibration or hesitation. This saved me over $500—what a relief! The engine now runs smoothly with no misfire symptoms.

Anonymous 2009-10-01 13:16

The check engine light returned after the initial fix, still showing PO305 and PO420 codes. I replaced the spark plugs and swapped the ignition wire from Cylinder 5 to Cylinder 7—no change in readings. No rotten egg smell or heat from the exhaust suggests the catalytic converter is likely not at fault. I'm now considering either a distributor cap/rotor issue or faulty fuel injector. How can I test these components? Which one would be easier and safer to diagnose first? Any advice would be helpful, thanks!

Anonymous 2009-10-01 21:38

I'd recommend completing the tune-up with a new distributor cap and rotor—especially for 1997 Vortec engines. Aftermarket parts can sometimes cause cross-firing even when brand-new. For reliability, consider OEM components from an auto dealer. As for injectors, they're difficult to diagnose without advanced scan tools and real-time data. Injectors on Vortec engines do fail frequently—so if the misfire persists after checking ignition components, injector testing should be considered next.