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2006 Buick Rendezvous P0420 Fault: Catalytic Converter Efficiency Below Threshold - O2 Sensor & Fuel System Diagnosis

Model: BUICK RENDEZVOUS Fault Code: P0420 Posted: 2015-03-05 22:32

I recently replaced both the catalytic converter and my pre- and post-cat oxygen sensors on my 2006 Buick Rendezvous V6 3.5L engine (approximately 151,000 km). Despite these replacements, I'm now seeing a recurring Check Engine Light with code P0420: 'Catalyst System Efficiency Below Threshold (Bank 1)'. I've started reviewing freeze frame data and am trying to interpret the readings. Here's what I have: - Fuel System 1: CL (Closed Loop) - Fuel System 2: NA (Not Available) - Load: 31.4% - ECT (Engine Coolant Temp): 84°C - STFT Bank 1: -0.8% (Slightly lean) - LTFT Bank 1: -7.8% (Running rich) - MAP: 43 kPa - Engine RPM: 645 - Park Adv: 10.5 - IAT: 1°C - MAF (g/s): 5.34 - TPS: 7.5% - Warm-up DTC Cycle: 12 - EVAP_VP: 73.00 Pa - Baro: 99 kPa - Cat Temp Bank 1 Sensor 1: 584°C - ECU Voltage: 14.218 V - Load Value: 25.9% - Equivalence Ratio: 1.167 (slightly rich) - TPS Relative: 3.1%, TPS B: 7.5% The engine is running within the 10% trim tolerance, but I'm consistently reading a rich condition. I'm concerned whether my replacement sensors were faulty or if there's an underlying issue with the fuel system. I've searched for troubleshooting guides, but most are either outdated or not specific to this model. Any advice on how to properly diagnose this P0420 code—especially using O2 sensor waveforms and ensuring proper pre/post-cat comparison? I'd greatly appreciate any insights or resources pointing me in the right direction.

Related fault codes

P0420

Comments (3)

Anonymous 2015-03-06 08:43

Only P0420 is present — did you install the sensors from a reputable brand like Buick or GM original equipment (OE)? Aftermarket sensors can vary significantly in quality and performance. Also, note that your 2006 Rendezvous has a crossover setup with only one catalytic converter (not dual), which differs from California emissions vehicles. For accurate diagnosis, you should compare the voltage waveforms of the pre-cat (Bank 1 Sensor 1) and post-cat (Bank 1 Sensor 2) O2 sensors using an OBD-II scanner that supports waveform analysis. Refer to reliable sources like https://www.obd-codes.com/p0420 for detailed explanations.

Anonymous 2015-03-06 10:08

I used Ac Delco original GM OEM oxygen sensors. One sensor was installed by a mechanic, who charged me $2 hours of labor for the replacement. I did the second one myself — it's a Canadian vehicle with standard emissions regulations.

Anonymous 2015-03-06 10:38

P0420 is notoriously difficult to diagnose because it involves complex engine control logic. The PCM uses a catalytic converter to reduce harmful exhaust emissions. It monitors this process using two oxygen sensors: one before (pre-cat) and one after the catalytic converter (post-cat). The post-cat sensor should show significantly lower activity than the pre-cat sensor when the converter is working properly. If the PCM detects that the post-cat sensor's signal remains outside an acceptable range for a sustained period, it sets DTC P0420 — indicating the catalyst has insufficient oxygen storage capacity. To diagnose this correctly: compare the voltage waveforms of Bank 1 Pre-Cat O2 Sensor (HO2S #1) and Post-Cat O2 Sensor (HO2S #2). The post-cat signal should be much less active than the pre-cat one. This comparison is best observed using a quality OBD-II scanner with waveform visualization. Crucially, ensure there are no other underlying issues or codes before focusing on this P0420 code — it may be triggered by fuel system problems, incorrect air-fuel ratio, or sensor malfunctions.