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152 Posts
Discussion Starter #1
1999 frc... Modified exhaust,,Kooks long tube road X pipe (no cats) "rear sensors removed" and Borla stinger tips with no mufflers.. yes its loud but I love it.

My question / problem is..check engine light.

I can clear the alarms but they always return. I assume because I have no rear sensors. the alarms are as follows.

1. 10-pcm--P0153 H
P1134 HC
P1154 H
2. SDM----B0026 HC
B1001 H

Im pretty sure the SDM alarms are due to the fact that the original steering wheel has been removed and an aftermarket one put in its place. So not to concerned about those right now.

Question is, do I have a bad sensor in front? And can I cheat the system and get around the rear sensor alarm for the missing rear sensors???

I need to mention that I have disconnected the front sensors during another problem I was chasing. I reconnected them but not 100 percent sure I connected them properly. In other words there are two male n two female connectors on each side of the exhaust right by the collector for the headers, like I said not 100 percent sure I got it right.

Can any one shed some light on 1. can I cheat the system for the missing rear sensors ? And 2. how can I be sure that I have the front sensors properly connected in the correct plug in? Thank You for any ideas You wanna share.

606 Posts
Engine codes you have say both front 02 sensors are not working correctly

P0153 O2 Sensor Circuit Slow Response (Bank 2 Sensor 1)

P1134 Heated Oxygen Sensor (HO2S) Transition Time Ratio Bank 1 Sensor 1

P1154 Heated Oxygen Sensor (HO2S) Transition Time Ratio Bank 2 Sensor 1

You cannot turn OFF the rear 02s, only turn off their error codes from tripping.

Bad idea not using the rears as they also are used by the PCM to compute the fuel trims

As per GM :

The rear oxygen sensor, located after the catalyst, is used for fuel trim corrections on OBD-II vehicles. By virtue of its location, the rear sensor is generally protected from high temperatures and much of the contamination that affects the front oxygen sensors.
In addition, the rear sensor sees exhaust gases that are equilibrated – they have already been converted by the catalyst so that there is very little residual oxygen.
This allows the rear sensor to respond to much smaller changes in exhaust gas oxygen content. In turn, it then possible for the rear sensor voltage to remain near the 0.45 volt switchpoint.

This characteristic allows the rear sensor to be used for fuel control. Under steady rpm and load conditions, the short term fuel trim bias can be adjusted so that the rear sensor voltage is maintained near the 0.45 volt switchpoint.
This ensures that the catalyst is getting a stoichiometric exhaust gas mixture, despite any shift in the front sensor switchpoint.
The rear fuel trim corrections are learned in KAM (Keep Alive Memory).
Internally, this system is known as Fore Aft Oxygen Sensor Control (FAOSC). Note that FAOSC learns and reacts very slowly because the catalyst, with its large/slow oxygen storage and release characteristic, is part of the control loop. Also, this system cannot be used with a "y-pipe" exhaust where a single rear sensor would try to adjust dual front sensors.

Rear O2s if you will are a fine tune of the commanded fuel flow but are very much part of the model used in the PCM's math to correct AFR for closed loop

This means running no rear O2s or Simms will effect how the PCM computes what the fuel trims are.

There is 2 02 wiring connectors per side on frame rail
Each side has 2 different shaped wiring connectors and the front and rear 02s have a matching connector shape so they cannot be connected in wrong UNLESS someone has hacked the 02 wires and used the wrong connector or incorrect wiring order.

Look at the stock 02 sensors and it will be clear which shape they have and match to the main wiring harness.

If you are wrongly using the rear 02 connectors for the fronts then they are on the wrong side of engine as the right rear connector is for the left rear 02 and same for left side is for right rear 02.

Using a OBD-II scanner would quickly tell you the state of the 02 sensors and wacked fuel trims would point to 02s in the wrong placement

152 Posts
Discussion Starter #3
O2 sensors

Thanks so much for the info. Very helpful. My dilemma seems to be the fact that I have no cats to effect an exhaust oxygen-gas mix. Its simply a straight pipe from the head exhaust port to the exhaust tip. Ive read about dummy plugs and my local tuner told me yesterday he can turn the rear sensors off for 150 bucks.

I don't want to put cats back on the car. It seems that I'm trying to trick the processor into simply ignoring the fact that I have no aft sensors on the exhaust system. The free flow exhaust is worth sacrificing the fuel tweaks I'm giving up." I think".:thud::thud:
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