Where is the sensor for code P0421, Mode $06, Component Id $05, TID $01?

The car: 2006 Boxster S with over 130,000 miles. Running perfectly fine overall. Only tracked once over 10 years ago; otherwise, it's my daily driver -- a mix of highway and stop-and-go during the week, long drives up in the hills on the weekends. The issue: * I get the infamous "P0421 - Warm Up Catalyst Efficiency Below Threshold" error, and per the below report from my OBDII code reader ( OBDLink LX ) apparently this code is from a single failure on "Mode $06 - Component Id $05 - TID $01". According to the OBDII app, the minimum value of this parameter is 2048, with no maximum. * The failure is intermittent: sometimes this parameter is around the 1200 - 1500 range, and frustratingly sometimes it fixes itself and is in-spec (2443 was one reading) Example excerpt from a recent code reader report: Mode $06 - On-Board Monitoring Component Description Value Minimum Maximum Units Result Component Id $05 TID $01 - Manufacturer Defined 1581 2048 N/A Fail Component Id $06 TID $01 - Manufacturer Defined 6636 2048 N/A Pass * About 18 months ago I did the basics: replaced the original gas cap, and replaced the back-end O2 sensors. No change/impact. * I've noticed it can stay fixed and/or it will fix itself when it's relatively cool weather (below 50's F), or paradoxically when it's really hot out -- in the 90's * It can sometimes stay fixed long enough to go through a whole drive cycle, thus clearing the CEL. Indeed, when I reset all the CEL codes with my OBDII reader I can often get through a drive cycle and the code/parameter/CEL stays in-spec for quite a few miles/days * The only info on this code I've been able to find (searching the internet -- nothing came up on this or other Porsche forums) is from a manual from a 911 Carrera, saying the code is used for "measuring the aging of the catalytic converters". I haven't been able to confirm if this is the same setup for a 2006 Boxster S. * My mechanic believes the only way to really fix this is to replace the corresponding cat's Bank. (And I can't say he's wrong -- the car will soon be 20 years old, after all; but naturally before I drop $$$$ I want to know more.) My hypothesis (and hope): the intermittent nature of this parameter going in-and-out-and-back-in spec. could simply mean that some sensor in-or-around the cat bank needs to be replaced -- or even just removed and cleaned. So back to the question posed in the title of this post: Where and what is the sensor behind P0421, Mode $06, Component Id $05, TID $01? Is there some wiring harness I can trace to find this sensor? Help! P.S. For reference, here is the full code reader report -- a snapshot taken during my commute to work (57 degree day, car was warmed up): Diagnostic Report Created by OBDLink - OBD Solutions​ www.obdsoftware.net ​ Date: 1/22/2026 10:34:06 AM VIN: < redacted > Year: 2006 Make: PORSCHE Model: Boxster Monitor Status Report ECU 11 Name Continuous Status Misfire Yes ECU has completed this test Fuel System Yes ECU has completed this test Comprehensive Component Yes ECU has completed this test Catalyst No ECU has completed this test Heated Catalyst No ECU does not support this test Evap System No ECU has completed this test Secondary Air System No ECU has completed this test Gasoline Particulate Filter No ECU does not support this test Oxygen Sensor No ECU has completed this test Oxygen Sensor Heater No ECU has completed this test EGR and/or VVT System No ECU does not support this test MIL On Number of Confirmed Codes: 1 Readiness Standard: CA - 2000 and newer (Gas) This vehicle is not ready for emissions testing. Reason ECU 11 The MIL is On Confirmed trouble codes have been detected Trouble Code Report ECU Code Type Status UDS Status Description ECU 11 P0421 Powertrain Confirmed N/A Catalyst 1 Efficiency Below Threshold Bank 1 ECU 11 P0421 Powertrain Pending N/A Catalyst 1 Efficiency Below Threshold Bank 1 Additional Information Description Value Units Malfunction indicator lamp (MIL) status On Freeze frame DTC P0421 Distance traveled while MIL is activated 554.88 miles Mode $01 - Powertrain Diagnostic Data PID Description Value Units SAE 0x03 Fuel system 1 status Closed-loop, using oxygen sensor feedback for fuel control SAE 0x03 Fuel system 2 status Closed-loop, using oxygen sensor feedback for fuel control SAE 0x04 Calculated load value 29.41 % SAE 0x05 Engine coolant temperature 186.8 °F SAE 0x06 Short term fuel % trim - Bank 1 6.25 % SAE 0x07 Long term fuel % trim - Bank 1 3.12 % SAE 0x08 Short term fuel % trim - Bank 2 7.03 % SAE 0x09 Long term fuel % trim - Bank 2 3.91 % SAE 0x0C Engine RPM 678.75 RPM SAE 0x0D Vehicle speed 0 MPH SAE 0x0E Ignition timing advance for #1 cylinder 3.5 deg SAE 0x0F Intake air temperature 86 °F SAE 0x10 Mass air flow rate 0.45 lb/min SAE 0x11 Absolute throttle position 13.73 % SAE 0x12 Commanded secondary air status Atmosphere/off SAE 0x13 Location of oxygen sensors Bank 1: Sensor 1, Sensor 2, Bank 2: Sensor 1, Sensor 2 SAE 0x15 O2 voltage (Bank 1, Sensor 2) 0.675 V SAE 0x15 Short term fuel trim (Bank 1, Sensor 2) 99.219 % SAE 0x19 O2 voltage (Bank 2, Sensor 2) 0.665 V SAE 0x19 Short term fuel trim (Bank 2, Sensor 2) 99.219 % SAE 0x1C OBD requirements to which vehicle or engine is certified OBD II (California ARB) SAE 0x21 Distance traveled while MIL is activated 554.88 miles SAE 0x34 O2 sensor lambda wide range (current probe) (Bank 1, Sensor 1) 0.997 SAE 0x34 O2 sensor current wide range (Bank 1, Sensor 1) -0.016 mA SAE 0x38 O2 sensor lambda wide range (current probe) (Bank 2, Sensor 1) 1.005 SAE 0x38 O2 sensor current wide range (Bank 2, Sensor 1) 0.008 mA SAE 0x56 Long term secondary oxygen sensor trim bank 1 -0.78 % SAE 0x58 Long term secondary oxygen sensor trim bank 2 -0.78 % Mode $02 - Freeze Frame P0421 - First Occurrence Description Value Units Freeze frame DTC P0421 Fuel system 1 status Closed-loop, using oxygen sensor feedback for fuel control Fuel system 2 status Closed-loop, using oxygen sensor feedback for fuel control Calculated load value 33.73 % Engine coolant temperature 188.6 °F Short term fuel % trim - Bank 1 -3.91 % Long term fuel % trim - Bank 1 0.78 % Short term fuel % trim - Bank 2 -4.69 % Long term fuel % trim - Bank 2 1.56 % Engine RPM 657.25 RPM Vehicle speed 0 MPH Ignition timing advance for #1 cylinder 8 deg Intake air temperature 96.8 °F Mass air flow rate 0.56 lb/min Absolute throttle position 14.51 % Commanded secondary air status Atmosphere/off Long term secondary oxygen sensor trim bank 1 -0.78 % Long term secondary oxygen sensor trim bank 2 -0.78 % Mode $05 - Oxygen Sensors Sensor Available Bank 1 - Sensor 1 Yes Bank 1 - Sensor 2 Yes Bank 1 - Sensor 3 No Bank 1 - Sensor 4 No Bank 2 - Sensor 1 Yes Bank 2 - Sensor 2 Yes Bank 2 - Sensor 3 No Bank 2 - Sensor 4 No Sensor Description Value Minimum Maximum Units Result Bank 1 - Sensor 1 TID $81 - Manufacturer Defined 15 0 255 Pass Bank 1 - Sensor 1 TID $82 - Manufacturer Defined 16 0 255 Pass Bank 1 - Sensor 1 TID $83 - Manufacturer Defined 15 4 255 Pass Bank 1 - Sensor 2 TID $01 - Rich to lean sensor threshold voltage (constant) 0.605 0 1.275 V Pass Bank 1 - Sensor 2 TID $02 - Lean to rich sensor threshold voltage (constant) 0.605 0 1.275 V Pass Bank 1 - Sensor 2 TID $07 - Minimum sensor voltage for test cycle (calculated) 0 0 0.45 V Pass Bank 1 - Sensor 2 TID $08 - Maximum sensor voltage for test cycle (calculated) 0.95 0.45 1.08 V Pass Bank 2 - Sensor 1 TID $81 - Manufacturer Defined 15 0 255 Pass Bank 2 - Sensor 1 TID $82 - Manufacturer Defined 16 0 255 Pass Bank 2 - Sensor 1 TID $83 - Manufacturer Defined 16 4 255 Pass Bank 2 - Sensor 2 TID $01 - Rich to lean sensor threshold voltage (constant) 0.605 0 1.275 V Pass Bank 2 - Sensor 2 TID $02 - Lean to rich sensor threshold voltage (constant) 0.605 0 1.275 V Pass Bank 2 - Sensor 2 TID $07 - Minimum sensor voltage for test cycle (calculated) 0 0 0.45 V Pass Bank 2 - Sensor 2 TID $08 - Maximum sensor voltage for test cycle (calculated) 0.925 0.45 1.08 V Pass Mode $06 - On-Board Monitoring Component Description Value Minimum Maximum Units Result Component Id $05 TID $01 - Manufacturer Defined 1581 2048 N/A Fail Component Id $06 TID $01 - Manufacturer Defined 6636 2048 N/A Pass Component Id $15 TID $02 - Manufacturer Defined 3936 1229 N/A Pass Component Id $16 TID $02 - Manufacturer Defined 4175 1229 N/A Pass Component Id $3C TID $02 - Manufacturer Defined 327 N/A 329 Pass Component Id $27 TID $02 - Manufacturer Defined 331 329 N/A Pass Component Id $2F TID $02 - Manufacturer Defined 38 N/A 58 Pass Component Id $3D TID $02 - Manufacturer Defined 327 N/A 329 Pass Component Id $28 TID $02 - Manufacturer Defined 331 329 N/A Pass Component Id $30 TID $02 - Manufacturer Defined 39 N/A 58 Pass Component Id $01 TID $03 - Manufacturer Defined 32000 4864 N/A Pass Component Id $04 TID $03 - Manufacturer Defined 30208 4864 N/A Pass Component Id $20 TID $05 - Manufacturer Defined 44 19 N/A Pass Component Id $29 TID $05 - Manufacturer Defined 3022 3021 N/A Pass Component Id $2A TID $05 - Manufacturer Defined 3664 3662 N/A Pass Component Id $2B TID $05 - Manufacturer Defined 3719 2458 N/A Pass Component Id $0D TID $06 - Manufacturer Defined 43680 40881 N/A Pass Component Id $0E TID $06 - Manufacturer Defined 43638 40881 N/A Pass Component Id $11 TID $06 - Manufacturer Defined 41 N/A 3009 Pass Component Id $12 TID $06 - Manufacturer Defined 75 N/A 3009 Pass Component Id $0A TID $09 - Manufacturer Defined 0 N/A 11 Pass Mode $09 - Vehicle Information General Information Description Value Vehicle Identification Number WP0CB29816S730119 Calibration ID - ECU 11 9876186240300000 Calibration Verification Number - ECU 11 EA1EB10C In-Performance Tracking ECU Counter Description Value ECU 11 0x00 OBD Monitoring Conditions Encountered Counts 5835 ECU 11 0x01 Ignition Cycle Counter 16800 ECU 11 0x02 Catalyst Monitor Completion Counts Bank 1 5523 ECU 11 0x03 Catalyst Monitor Conditions Encountered Counts Bank 1 5807 ECU 11 0x04 Catalyst Monitor Completion Counts Bank 2 4984 ECU 11 0x05 Catalyst Monitor Conditions Encountered Counts Bank 2 5807 ECU 11 0x06 O2 Sensor Monitor Completion Counts Bank 1 5524 ECU 11 0x07 O2 Sensor Monitor Conditions Encountered Counts Bank 1 5835 ECU 11 0x08 O2 Sensor Monitor Completion Counts Bank 2 5597 ECU 11 0x09 O2 Sensor Monitor Conditions Encountered Counts Bank 2 5835 ECU 11 0x0A EGR and/or VVT Monitor Completion Condition Counts 13308 ECU 11 0x0B EGR and/or VVT Monitor Conditions Encountered Counts 5835 ECU 11 0x0C AIR Monitor Completion Condition Counts (Secondary Air) 3168 ECU 11 0x0D AIR Monitor Conditions Encountered Counts (Secondary Air) 3725 ECU 11 0x0E EVAP Monitor Completion Condition Counts 977 ECU 11 0x0F EVAP Monitor Conditions Encountered Counts 2580

Those aren't real codes, but rather the code reader is unable to decode it. You need a scanner that is Porsche -specific ( OBDLink doesn't have an offering for that, though they do cover other brands). There are a few options talked about on this forum, so see what works for you. I, personally, got the Foxwell NT530 . Another option is to go to a shop with a reader that can do this for you... they may charge you for it. I don't think the scanners from auto parts stores will do this, but you can try those as well (Autozone, for example, does scans for free...) Once you get that scanner, you can re-read the codes and see what's really going on. Until then, you'll just be guessing.

Update: Many thanks schwinn for the recommendation of a reader that can get at more Porsche -proprietary data! I've bought the recommended Foxwell NT530 reader, and lo' and behold the thing-y with the failed parameter is labeled " Exhaust gas sensor monitor bank 1 - sensor 1($01)" Per the below screen shot (and consistent with the other code reader's data), one "half" of this sensor group reports a failure -- with a Value of 1398 in this snapshot. As you see this is below the required minimum value of 2048. (FWIW, I've seen this value fluctuate up to 1600 or so; and per the above into, depending on hard-to-pin-down conditions it can fix itself and go into spec.) Conversely, the other "half" of this "Exhaust gas sensor monitor" pair is within spec (and always has been -- this has never had a failure) Thusly, I ask: 1 - Where are these "Exhaust gas sensors" located? (Are they right on the exhaust manifold coming off the engine block, or way down on the tail pipe)? 2 - Any idea what these numbers are measuring and/or why is the minimum of this parameter 2048 vs. anything else? (It is an encoded/transformed temperature measurement? A voltage in millivolts which can be translate to a temperature? Measurement of the concentration of some chemical coming from the combustion process?) Help!

Did you get the Porsche add-on and install it to the device? Because it looks like it's still not reading the codes completely... I haven't seen any properly read codes with the $ in them...

Yes. The Porsche "app" is installed in my Foxwell NT530 reader. And to elaborate on how I arrived at the above images: 1 - Click on the OBDII app 2 - Click "Autoscan" and/or come to find out the Boxster uses the ISO9141-2 protocol. 3 - You get a "Codes found" message. Click --> 4 - Go into menu choice 5 "On board monitor" 5 - Scroll down the list of manufacturers until you hit "36 - PORSCHE " 6 - Keep clicking into menu choices and you get to the above screens. Oddly, drilling down for codes with the Porsche app itself on the device only gives you the top level "P0421" code.

I have also been trying to solve the intermittent P0421. the error code I don't see on your report tells you which bank is causing the problem. in other words, B1 or B2 catalytic converter . B1 on the right from rear, B2 on the left from rear. Porsche reversed it which confuses the hell out of everyone including shops and parts stores. Porsche treats the left as the passenger side, and the right as the driver side in a LHD car. note that this is flipped around in a Carrera . it depends on which orientation the engine is facing. this is just damn stupid imo. catalytic converters produce a voltage value from 0.0 to 1.0 V representing efficiency. (apologies: I meant to say O2 sensors here). B1 in my car is returning a cat value of 0.5 to 0.6. B2 is reporting 0.75, consistently. this tells me that the B1 catalytic converter is failing.

Apologies for not updating this sooner ... In the end, no one could answer the original question (Where is this sensor located?) -- not even my trusted mechanic, but he, gear head friends, and Grok all agreed the low number / intermittent behavior was due to a bad cat bank. Hence, I finally decided to get the new cat part and have my mechanic install it, and I'm relieved to report it worked! Now the "Component Id $05 TID $01 - Manufacturer Defined" parameter reads consistently over 18,000! (Yes, 10x what the failed number was.) Plus, everything else remains within spec., and [knock on wood] the car is running as well as ever. Further apologies Fan987, I can't recall which bank this was -- but the mechanic knew.

edit: I misunderstood your reply. your mechanic just replaced the O2 sensors .

" exhaust gas sensor" is an O2 sensor . why does your car have three of them? that's weird. there's just an upstream and a downstream O2 sensor on each bank, for a total of 4.