Intermittent Service Engine Light LT1

The Service Engine Soon light in my 96 RMW comes on sometimes after idling a bit. It seems to happen more frequently when it's warming up. I've checked for codes on the heater control but it doesn't come up with anything. I've read that some malfunctions don't "save". It has 1 new downstream O² sensor on the left side. It's a Bosch and I've read some not so favorable things about them here recently. I also have 1 broken exhaust manifold bolt on the left rear going into the head. It doesn't have much of an audible leak, but I can hear the slightest ticking when it's warming up. I've disconnected the battery to clear the light and it might take a day or two or even up to a week before it trips it again.

I'm wondering if that slight leak might be screwing with the fuel trim and turning on the light.

Might a real OBDII scanner be able to find a code that the heater control doesn't?

What else might be causing the light and what do I do too go about tracking it down?

There are a few codes that are self-resetting. However, the light goes out by itself when that happens. I don't know how complete the climate control diagnostics are on the 96, as the 96 is OBDII. There may be some new ones that don't show up on a system originally designed to be compatible with OBD 1. This is the procedure for an OBD1 car, I've never heard that the OBD2 cars are any different:

Start the car.
Press and hold the "temp up" and "OFF" buttons simultaneously until the display shows all segments lit.
Release the buttons. Display should read -00. This is the PCM diagnostic codes pointer.
Press the "middle" air distribution button. Display should show the first trouble code. A 3 digit code starting with a "1" is a history code. A 2 digit code is a current code.
Press "OFF" while a code is displayed to clear it.
Press "temp up" or "temp down" to scroll through multiple codes
Press "middle" again to return to the diagnostic modes pointer. Display should read "-00" again.
Press "temp up" or "temp down" to scroll through the other pointers. -00 is PCM, of course. -01 is the BCM, -02 is HVAC, -03 is Airbags, -04 is ABS. Press "middle" to display any codes associated with the respective pointer, as above.

Assuming that you are doing the above and not getting any meaningful result, then yes, I think you will need a code reader or even better, a scan tool.

The difference between a code reader and a scan tool is that a code reader does nothing more than spit out any codes. A scan tool, however, can also read the actual datastream of the various sensors and outputs from the PCM to various devices. Some scan tools can actually command the PCM (or ABS, etc.) to take certain actions for diagnostic purposes.

Once upon a time, a scan tool was a very expensive item, out of reach of most backyard mechanics. However, these days there are some reasonable alternatives: used professional tools on eBay, homeowner quality tools from companies such as Innova, software and adapters for your laptop, and Chinese clones of professional tools. Still 2 to 4 times the cost of a code reader, but well worth it in the long run, IMHO.

I'm going to guess you are in Indiana from your nickname, but if I'm wrong and you happen to be in the Toronto-Muskoka area, I'd be happy to plug one of my scan tools in for you.

Nothing wrong with Bosch o2 sensors

Without a code, hard to tell you what is going on.

When I had intermittent codes, most of them tied back to the air system.
It would happen randomly, go away for a few weeks, then come back.
After I removed it and had it programmed out, no light again.

I had the broken bolt in the same spot. I had fixed that at the same time, but don't think it was related

Ok, so I drove over to the Twilight Zone and had them scan it. P0161 - Bank 2 Sensor 2. I imagine it's just the sensor instead of a wiring problem. I'll test it first though. I already know that sensor is kind of a pain to pull because I inadvertently pulled it when I did the first one thinking it was bank 1. The sensor came out without a struggle but the wiring connector is up between the tailshaft and tunnel and hard to reach. Since it's the downstream one it really doesn't affect anything. It's just annoying when the light comes on. Thanks for the tip on clearing the codes with the climate control. I only knew how to access them so that'll save me from disconnecting the battery until I get a new one in.

Any idea why the climate control would not display a code for this side when it did for the left side? Both are after the converter.


I need to do some research and then invest in a scanner. The only other "new" car in my stable is my wife's '02 Chrysler (what can I say? she had it before we were married.) Eventually it will need to be replaced with something newer and a scanner would probably come in handy.

71Novaguy, were you able to remove the bolt broken manifold bolt in the car? Mine's broken off flush about with the head.

Thanks for the help guys.
Andrew

P0161 is specifically related to the heater performance rather than the signal performance of that sensor. It is set when the PCM decides it is taking too long for the sensor to warm up and start responding. The most common cause is a failed heater within the sensor but could also be caused by a wiring problem in the power supply to the heater element or by an exhaust leak -- not because the leak is affecting the fuel trim, but because it is affecting the temperature of the exhaust. It has to be a pretty good size leak to have much effect though.

It is theoretically possible that the code could pop up due to faulty fuel pressure, fuel injector(s) and/or intake air leaks, but those should set a raft of other codes as well, so I would consider those possibilities to be highly unlikely.

Pull the connector and probe the two heater wires for power (your FSM has the connector pin-out). If you have power at the connector, then swap sensors. If the problem swaps sides then the problem is the sensor. If it doesn't then the problem is probably the exhaust leak.

Couldn't not fix with engine in the car without removing the manifold. Even then, needed an extractor.

The way I understand it, the ECM can throw that code if the O² sensor isn't reading what it expects after a certain amount of time has passed after start up. Losing power to the heater circuit would do that. That might explain why it trips when I'm warming the car up but less likely when I just get in and go.

I don't have a FSM or even a Haynes manual yet. I did a little investigating on which wires to test for power. These are the wire colors on the sensor in order across the connector.

AC Delco
Purple- Sensor signal
Tan - Sensor ground
Brown - Heater ground
Brown - Heater 12v keyed

Bosch
Black - Sensor Signal
Grey - Sensor ground
White - Heater ground
White - Heater 12v keyed

I tested it and I have power to the connector. It's snowing here now. Instead of swapping sensors I just ordered a new one to replace it. Amazon had Bosch sensors priced at half of what El Vato Zone did and free shipping to boot.

Thanks again for the help.

Nova Guy, I'm not sure I follow you. Did you have to pull the engine or just the manifold to get to your bolt? I'm cool with the latter, but not the former.

I could have reached the bolt in the car if I removed the manifold. But I had the engine out to fix leaks anyways

I replaced the oxygen sensor a week ago and haven't had the service engine light come on since then. All 4 of the sensors were AC (presumably 80k originals) and now the rear 2 are Bosch. Should I expect the front sensors to go bad in the near future? If so I'll get some to have on hand. Should they just be replaced as a matter of course?

The rear O2s are only there to tell the PCM that the cats are functioning, the fronts are for setting the mix. When you do the fronts, put ACs back in. 71nova likes Bosch... but he might be the only one. No, don't expect them to go bad instantly, just do them when you are doing some maintenance.

I usually try to use OEM parts when I can. Since the downstream ones basically just satisfy the ECM to prevent the engine light from coming on (no emissions testing)I decided to get the cheaper parts.

I noticed that a picture of the AC Delco 2131325 sensor has the sensor tip with holes in it. The Bosch's that I put in and the AC's that I removed were the kind with 3 slots on the tip. It appears that all 4 sensors are the same part number so I presume the upstream ones will look like the rear ones I removed. Is your recommendation to use AC O² sensors based on a real world bad experience with other manufacturers? Or just a predilection towards OEM parts?

I think most of the people on here just follow along. I use what is available and what works. I have never had issues with Bosch O2's. I've used them on all my cars when replacing 02's. That being said I haven't ever used a AC delco 02 sensor, so I cant say what lasts longer, but I've never had to go back and replace a Bosch 02 sensor. I have, however, used AC delco spark plugs, and within 10k miles, I was replacing them again. Went to the basic autolight coppers, and no issues again.

I'm not against AC Delco, they make great batteries. Matter of fact, after almost 8 years, I finally replaced the battery in my Silverado with another delco battery. Its better to gain some experience than live on other preconceived notions.

Bosch O2 sensors use a different calibration curve than all other O2 sensors, and only work 100% with EFI systems calibrated to use Bosch sensors. I would recommend using Denso or Delco o2 sensors for best results.

mine isn't calibrated to bosch sensors. its oem tune using bosch sensors and no issues, nearly 10k miles later. (know its oem tune as I had it in at the dealer to have the air pump deleted).
They are direct replacement sensors, not the make your own harness type.