Help with P0135 & P0420 SES codes on 97 Firehawk LT1 M6

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prix57
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Help with P0135 & P0420 SES codes on 97 Firehawk LT1 M6

Post by prix57 » Sat Jun 15, 2019 3:18 am

My SES light has been on for a while now, I checked it and it's giving me the following codes:

P0135 - O2 Sensor Heater Circuit (Bank 1 Sensor 1)

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

I was hoping to get some assistance on getting these SES codes fixed. It's for my 97 Firehawk LT1 M6, it is all stock.

Should I just replace the O2 sensors or will I also need to replace the Catalytic converter?

tazman71
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Re: Help with P0135 & P0420 SES codes on 97 Firehawk LT1 M6

Post by tazman71 » Mon Jun 17, 2019 12:09 pm

Sounds like the first 02 sensor it having issues. Start with that one. It controls the air fuel. The secondary is just telling you that after the cat that it is not in spec. If your miles are high, might consider changing both, but only the first one will affect how it is running.
02 Firehawk TA #1237
Black/Black, 1LE, Chrome wheels, Auburn
95 Firehawk #361
Hurst, TOTL, FLP headers, 4.10 Strange 12 bolt, UMI suspension, LT4 HOTCAM, ported stock heads, 52mm Throttle Body, ATI Procharger,
70 Chevelle SS 396
70 Z28 Camaro in pieces

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NBrehm
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Re: Help with P0135 & P0420 SES codes on 97 Firehawk LT1 M6

Post by NBrehm » Wed Jun 19, 2019 5:56 pm

Change the pre-cat, drivers side sensor to start. I always suggest NTK since they are vented they hold up a little better.
-Nick
2001 Formula Firehawk #293

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NBrehm
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Re: Help with P0135 & P0420 SES codes on 97 Firehawk LT1 M6

Post by NBrehm » Wed Jun 19, 2019 5:57 pm

I just noticed the LT1 part of that, I've never had one so I don't know if it has 2 sensors up front, if not Sensor 1 should still be the one in front of the CAT
-Nick
2001 Formula Firehawk #293

tazman71
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Re: Help with P0135 & P0420 SES codes on 97 Firehawk LT1 M6

Post by tazman71 » Thu Jun 20, 2019 12:21 pm

As far as LT1 goes up to 95 they only had a single O2 sensor. In 96 and up they have at least 2 sensors. 1 before and 1 after the cat. Depending on year and how many catalytic converters they have
02 Firehawk TA #1237
Black/Black, 1LE, Chrome wheels, Auburn
95 Firehawk #361
Hurst, TOTL, FLP headers, 4.10 Strange 12 bolt, UMI suspension, LT4 HOTCAM, ported stock heads, 52mm Throttle Body, ATI Procharger,
70 Chevelle SS 396
70 Z28 Camaro in pieces

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