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1999 Mazda Protege ES P0171 and P0420: How to Fix Lean Condition Before Replacing Converter

Model: 1999 mazda protege es Fault Code: P0171 Posted: 2007-06-19 20:36

I own a 1999 Mazda Protege ES with a 1.8L engine, automatic transmission, and approximately 156,000 miles. When I first purchased the vehicle, it displayed a P0171 code (System Too Lean). I replaced the upstream O2 sensor, air inlet tube, spark plugs, air filter, and cleaned the MAF sensor. After these repairs, the car passed OBD2 diagnostics in North Carolina with no issues. However, after driving about 1,500 miles, the MIL reactivated and now shows both P0171 (System Too Lean) and P0420 (Catalyst Efficiency Below Threshold). All replaced parts were factory-original Mazda components. Mazda service now recommends replacing the catalytic converter and the EGR boost sensor. The car runs smoothly with no driveability issues and achieves excellent fuel economy. I'm concerned about addressing the lean condition permanently before installing a new converter. What specific checks or repairs should I prioritize? Is there any way to resolve the P0171 issue without replacing the catalytic converter? Thanks for your insights.

Related fault codes
P0171P0420
Comments (3)
Anonymous 2007-06-20 08:56

Intake gaskets and EGR valves are common causes of P0171 on Mazda models, especially in older vehicles like the 1999 Mazda Protege ES. These components should be inspected thoroughly if the code persists.

Anonymous 2007-06-20 15:27

The intake gasket is likely leaking. When removing the intake manifold, are there any preventive maintenance steps you should take—such as cleaning EGR ports or valves? Also, could the P0420 be a false reading caused by prolonged lean conditions, or does it indicate an actual catalytic converter failure?

Anonymous 2007-06-21 06:50

It's difficult to determine if the P0420 is truly indicative of converter failure or just a symptom of the underlying lean condition. The only definitive way to verify this is by addressing and repairing the intake system—especially the gasket and EGR components.