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98 Gs400 Emissions Fail High Hc Co But Passed Nox

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Help Needed with diagnosis of emission failure and cont'd ODB code P0125, fail to closed loop.

Hi There,

I hoping to find someone who may have experienced the same type of problem or someone knowledgeable in the witchcraft of emissions interpetation/Lexus codes. I and a number of people I have consulted have come up empty with solutions apart from replacing the entire fuel system / emission system / catalytic system.

1998 gs 400 emission failed due to ASM2525 HC ppm limit 52/180 value, idle limit 150/230 value, CO% limit ASM2525 limit 0.29/4.30 value, idle CO% limit 0.70/4.01 value. NOx ppm passed.

ODB codes history

First set of codes were PO125 temp for closed loop, P0150 B2S1 circuit malfucntion, P0153 B2S1 circuit slow, P0171 system lean, P0150 B2S1, P0153 B2S1.

Reset ECM, replaced B2S1 O2 sensor (original Denso with Delphi OEM 4 wire LTW.7595, 0938T1 coding on sensor), replaced cooling system thermostat WITH OEM.

Emission failed again with codes again PO125 temp for closed loop, P0150 B2S1 circuit malfunction, P0153 B2S1 circuit slow ONLY. HC and O2 values consistent with first result. Reset ECM, road test until CEL and checked code, only one P0125.

This is first emission fail for a routinely maintained GS 400. Mileage is high but was also on previous 2 year emission pass with flying colours. Your expert advise would be greatly appreciated as only other thoughts are CAT replacement/injectors etc which would be costly mistake to make. HELP!!

Trim Values at idle

Ignition advance - 11.0 deg

Injector Pulse Width - 2.69 msec

Long Term FT Bank 1 - 8.59%

Long Term FT Bank 2 - 15.62%

Mass Air Flow 4.7 g/s

O2 Sensor 1/2 Volts - 0.61 V

O2 Sensor 2/1 Volts - 0.00 V

Short Term FT Bank 1 - -0.78%

Short Term FT Bank 2 - 19.53%

Total FT Bank 1 - 1.00

Total FT Bank 2 - 1.00

DTCS - 2

Ignition Advance 12.0 deg

Long Term FT Bank 1 - 8.59%

Long Term FT Bank 2 - 15.62%

Mass Air Flow 4.4 g/s

O2 Sensor 1/2 Volts - 0.50 Volts

O2 Sensor 2/1 Volts - 0.00 Volts

O2 Sensor 2/2 Volts - 0.87 Volts

O2S B1S1 Lean/Rich 0.00 msec

O2S B1S1 Lean/Rich 0.00 msec

O2S B2S1 Lean/Rich 0.00 msec

More info

Engine Speed 736 RPM

Coolant Temp 203 degF

O2 Sensor 1/1 Volts 0.67 Volts

Vehicle Speed Sensor 0 mph

Calc Engine Load 14%


Ignition Advance 12.0 deg

Long Term FT Bank 1 8.59%

Long Term FT Bank 2 15.62%

Mass Air Flow 4.2 g/s

O2 Sensor 1/2 Volts - 0.19 Volts

O2 Sensor 2/1 Volts - 0.00 Volts

Please help! Apart from beginning the replacment of components without true justification that they are faulty, I intend to check wiring harnesses carefully and ECM. Thanks!!

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your getting the p0125 because of the o2 sensor not reading at the ecu. the sensor that should've been replaced is the first sensor on the passenger side. If this is the sensor that was replaced then trace the wire from the sensor to the ecu. p0125 has nothing to do with coolant temp even though thats how it reads.

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Hi Im Back,

Thanks Morgan for your reply. Yes you were correct in your answer to replace the passenger side O2 sensor, but Lexus does not like Delphi O2 sensors. Despite a guarantee from the supplier of OEM equivalent the Delphi sensor did not work and caused considerable headaches as a result. So a word to those that seek an answer, do not use Delphi in place of Denso. They will not work despite being a third of the price. Well it was worth a try. Delphi sensor was 125.00 CDN vs 320.00 for Denso from Toyota/Lexus dealer supplied here in Toronto Canada. After much searching through this forum and other Lexus forum's I purchased 2 Denso aftermarket O2 sensors from know the rest for 130.00 US + 30 shipping and GST 22.00 (SO LESS THAN 200 CDN. for the 2) They shipped via USPS with tracking so that I could watch the package take 2 days to get to Canada Customs and spend the next 7 getting cleared and delivered via Canada Post. So if you can wait I highly recommend this source for Denso replacement 02 sensors.

If you are getting the P0125 code continually and the voltages are similar to the above, Bank 2 vs Bank 1, 2:1 ratio both of your pre-cat sensors need replacing. They must be replaced with 234-4169 part.

If NOX values are low and/or in limit, the catyltic convertors should still be OK. Getting the right replacement O2 sensor is extremely important.

Just thought I would update forum with results/resolution.

I have to add having a patient understanding mechanic who also would rather replace only what needs to be replaced vs replacing item after item after item until issue is resolved is worth his weight in gold. I am glad I've got him!!

Hope this helps someone else.


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