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P0420/P0430 Fault Diagnosis and Emissions Testing Solution for 2005 Lexus GX 470 V8

Model: Lexus V-8 Fault Code: P0420 Posted: 2012-10-18 19:44

This is my first post on the forum, and I'm grateful for the community's support. I've been using an Actron CP9180 scanner to capture detailed diagnostic data, including Diagnostic Monitor Tests. If any parameter or specific reading could help identify a root cause, please let me know—I’ll share it immediately. Vehicle: 2005 Lexus GX 470 V8 Issue: Persistent DTCs P0420 and P0430 (Catalyst Efficiency Below Threshold — Bank 1 and Bank 2). The MIL is illuminated, but the vehicle runs smoothly with no noticeable loss in performance or fuel economy. Background: - I recently replaced the alternator due to failure. During installation, several power interruptions occurred due to low battery states, leading to multiple unintended ECM resets. - Shortly after, DTCs P0441 (EVAP Purge Flow Fault) and P0455 (Large EVAP Leak) appeared. A new fuel cap resolved these issues. - Since then, the P0420/P0430 codes have emerged—no recurrence of evap faults even when reverting to the old gas cap. Emissions Testing Experience: I attempted a state emissions test (NV), which failed because the OBD II system was not ready. The testing authority rejected the vehicle due to 'not ready' status for: OBD Catalyst, OBD Evap, OBD Secondary Air, and Oxygen Sensor systems. After clearing P0420/P0430, the codes reappear shortly after driving, with the MIL turning on again. Key Questions: 1) How is it possible to have two catalysts fail simultaneously—or two exhaust leaks (no audible signs)—or for both downstream oxygen sensors to degrade at once? The timing coincides with the alternator replacement and the resolution of evap issues. Is this a coincidence or a systemic issue? 2) Should I expect additional specific codes if one or more upstream/downstream O2 sensors are faulty? Goal: Due to expired registration from emissions failure, I need to pass testing as quickly as possible. While addressing the root cause is essential, I’m seeking practical techniques to complete a valid drive cycle without triggering DTCs. I’d appreciate any suggestions for further diagnosis or troubleshooting steps. Thank you in advance! Additional Data: - Freeze Frame (after re-triggering codes): Engine idle at 710 RPM, coolant temp 195°F, IAT 132°F, ambient air ~80°F. - Catalyst inlet/outlet temperatures: ~295°F (inlet), ~525°F (outlet) — balanced across both banks. OBD system shows a 273°F temperature rise with identical readings on left and right sides (892°F vs. 619°F). - O2 Sensor Readings: - O2S12: 0.740V (up arrow) - O2S22: 0.760V - O2S11: 3.254V (EQ Ratio 1.000, EQ Ratio 21 = 0.997) - O2S21: 3.249V (down arrow) - Diagnostic Monitor Test Results: - B1S1: Voltage range 0.199–0.848V; current range 104.003–33.007mA - B1S2: Voltage range 0.175–0.722V; current range 0.605–0.722V (measured) - B2S1: Voltage range 0.199–0.862V; current range 104.003–43.007mA - B2S2: Voltage range 0.117–0.800V; current range 0.605–13.437mA - Catalyst B1 Test: Measured voltage 14.218V (max 19.960V) - Catalyst B2 Test: Measured voltage 8.046V (max 19.960V) - STFT/LTFT values: ST FTRM1 = -1.0%, LT FTRM1 = 4.6%; ST FTRM2 = -1.0%, LT FTRM2 = 5.4% - EWMA Misfire Count: Maximum value 65,535 (all cylinders), MEAS and MIN values are zero—suggests a history of rich/lean conditions or misfires. Note: Alternator replacement involved front KDSS sway bar removal with hydraulic servo cycling; battery swap included multiple on-road sessions. No other modifications performed. EVAP codes appeared before catalyst codes, never simultaneously. VSC warning lights appear when MIL is on but no specific DTCs for VSC system. Current Plan: Considering O2 sensor replacement due to inconsistent post-cat readings and potential misfire history. Seeking confirmation on correct part numbers (especially pre-cat) and whether universal Denso sensors are reliable. Part References: - Pre-cat Denso 234-4209 (Amazon, ~$110; possibly plug-in) - Post-cat Denso 234-4209 (universal, $37.90 with crimp connectors) - Inquiry: Is there a universal pre-cat sensor available at a lower cost? Are OEM or Denso sensors recommended over generic universal versions?

Related fault codes

P0420P0430

Comments (7)

Anonymous 2012-10-19 09:48

NO - clearing the DTC erases the monitor 'ready' status, operating the vehicle thru drive cycle to set monitors would test effecency of cats. All at once would have me dbl checking and questioning also- are there any PENDING codes? is scanner capable of P1xxx codes? the only code is p0420-p0430? what does the scanner tell you - freeze frame? look at your o2 sensor readings - bank 1 pre and post, and bank 2 pre post

Anonymous 2012-10-19 12:30

KEV2, I appreciate you looking at issue. I'm not 100% positive about the 1xxx codes, but the Actron CP9180 data sheet states it can read "enhanced codes" for GM/Ford/Chrysler ... so I guess that's a "no" for Toyota/Lexus. I'm sure your response would be "hook up with a scanner that can." I will, but I'm away from the vehicle for over a week, and need to formulate a quick plan of action for when I get my hands on it again. The scanner does freeze frame. FYI, this freeze frame was taken shortly after a drive cycle re-triggered codes 420/430, engine running, sustained idle @710 RPM, coolant 195 degrees, IAT 132 degrees, ambient air temp around 80 degrees. Laser temp samplings on each cat were near balanced, about 295 inlet, 525 outflow (each shot at the cat's narrowing taper area, roughly in between exhaust tube diameter and max cat diameter. OBD cat temps show a 273 degree rise and are identical left/right: 892 vs. 619. Discover more Ram Pickup DODGE TRUCK Fuel I haven't figured out how to post a pic of the scanner screen here, so here are the O2 readings: O2S12 V 0.740 (up arrow adjacent), O2S22 V 0.760, O2S11 V 3.254 (the display shows other information before the last sensor reading. In case it's meaningful, that is: EQ RATIO11 1.000, EQ RATIO21 0.997), O2S21 V 3.249 (down arrow adjacent). I also have diagnostic system monitor test results, which I think I selected individually in real time, not freeze frame: O2 Sensor B1S1 Test $8E Module $00 MAX: 19.898 V MEAS: 0.848 V MIN: 0.199 V O2 Sensor B1S1 Test $91 Module $00 MAX: 151.011mA MEAS: 33.007mA MIN: 104.003 mA O2 Sensor B1S2 Min V for Cycle Module $00 MAX: 0.195 V MEAS: 0.175 V MIN: 0.000 O2 Sensor B1S2 Max V for Cycle Module $00 MAX: 1.190 V MEAS: 0.722 V MIN: 0.605 V O2 Sensor B1S2 Test $8F Module $00 MAX: 7.902 MEAS: 4.765 MIN: 0.000 O2 Sensor B2S1 Test $8E Module $00 MAX: 19.898 V MEAS: 0.862 V MIN: 0.199 V O2 Sensor B2S1 Test $91 Module $00 MAX: 151.011mA MEAS: 43.007mA MIN: 104.003 mA O2 Sensor B2S2 Min V for Cycle Module $00 MAX: 0.195 V MEAS: 0.117 V MIN: 0.000 O2 Sensor B2S2 Max V for Cycle Module $00 MAX: 1.190 V MEAS: 0.800 V MIN: 0.605 V O2 Sensor B2S2 Test $8F Module $00 MAX: 7.902 MEAS: 13.437 MIN: 0.000 Catalyst B1 Test $A9 Module $00 MAX: 19.960 MEAS: 14.218 MIN: 19.921 Catalyst B2 Test $A9 Module $00 MAX: 19.960 MEAS: 8.046 MIN: 9.921 Thanks for taking the time to crunch all these numbers. Please let me know if there's any other info which would be useful. - Rich

Anonymous 2012-10-19 14:24

something stands out and I would ask- what is STFT and LTFT..... did you do anything else along with alternator swap...

Anonymous 2012-10-19 19:22

kev2 wrote: something stands out and I would ask- what is STFT and LTFT..... did you do anything else along with alternator swap... I think the format is a bit different, but I believe this is what you asked about:ST FTRM1 % -1.0, LT FTRM1 % 4.6, ST FTRM2 % -1.0, LT FTRM2 % 5.4 The only work that was done aside from the alternator swap was a battery swap, which included several occasions to stay on the road while waiting for the alternator to arrive. At least once it was driven to electrical exhaustion, wherein diminished engine output due to presumed ignition/fuel delivery issues caused my wife to pull over and become stranded. The alternator R&R required dropping the front KDSS sway bar - I know it's hydraulic servo cycled a couple of times through the process. Intake air filter and oil change somewhere in there too. Otherwise I cannot think of any other work that occurred. I will emphasize that none of these codes appeared until shortly after the alternator swap, and the evap codes preceded the catalyst codes, and they never appeared simultaneously. The on-dash VSC annunciators tend to accompany these codes, but as I understand it there are no DTCs specific to the VSC (or at least my scanner cannot read them) - but others have said that the GX 470 VSC will shed with the MIL illuminated for a variety of reasons that do not imply an actual failure within the VSC, it's a sympathetic reaction. Perhaps you know the details on that. As an aside , checking EWMA Misfire Cnt on each cylinder shows a MAX of 65535 cnts, and the values for "MEAS and MIN" are 0 cnts; the number is the same for 1-8. Not sure what that means, but I know catalyst failures tend to correlate with a history of rich/lean A/F ratios and/or misfires. That's all I can think of for now ... Thanks again, Rich

Anonymous 2012-10-20 12:13

My Apology - I need to 'rethink' there is a differance in o2 sensor and AF sensor data- being old I am always thinking o2 - What I thought was bad readings post cat sensors- typical of bad slow lazy sensors, will rething and advise- sorry for confusion

Anonymous 2012-10-21 13:31

Well, I'm definitely leaning toward 02 sensor replacement, given the present options. This is sort of a model specific question, but has a generic element too: should the downstream sensor be considerably less than the upstream? I'm finding the post-cat sensor for less, despite it being 4-wire heated. Seems opposite of what the price differential should be. But I have identified both plug and play Densos (OEM supplier) and a universal for the post-cat. The one Denso I have found for precat is vaguely identified, not yet sure if it's a plug-in or requires wiring. Customer reviews and other sources swear the Densos are the identical sensor in universal form, it's just either with or without plug. The negative comments about the universal versions all have to do with wiring issues, including one gent who probably overheated the probe while soldering (crimp connection recommended, but he went the "extra mile"). I've got a Denso 234-4209 located on Amazon for $37.90. Said to be identical to OEM except it is universal in the sense it must be wired to existing plug. Comes with crimp connectors and shrink tubing for that purpose. The pre-cat Denso I have located is a 234-4209 for at least $110. Not yet verified, but it could be a direct plug in. Still trying to find a Denso that would require splicing, if the price difference commensurate like the post-cat. Since I am entering the realm of parts-changing without an absolute diagnosis, it's important to keep the cost down as long as the sensor is right. I don't mind the wiring. Any comments on the universal Denso sensors, or a part # for the pre-cat that represents the universal version? I like the idea of a sub-$200 job for all 4 sensors as long as they are legit. Especially if I have to move onto replacing cats after that!

Anonymous 2012-10-21 13:57

my confusion might help explain the price differance - I have been told*- vehicle uses AF sensor pre cat, o2 sensors post cat... denso is a good option after OEM (toyota) , you do not want universal one size dose not fit all... * have not been at shop to dbl check manuals for this brain fart I am suffering.