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Too Many #$%^& Codes

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Guys and Gals,

So, I just got finished the head gasket/timing belt job from hell, all started due to bad knock sensors (see prior posts). Got it on the road, purring nicely and within a few miles it threw a code. This was the first (of many) an 0135 (02 sensor heater circuit, bank 1 sensor 1).

So I replaced that 02 sensor with another (yanker yard) one (not too difficult, if you have a long enough arm and can swing a wrench w/ one hand...took only 10 min). Cleared it and went driving w/out a CEL. So the next day, I took it out and bam, 2nd CEL, turned out to be a 0402, EGR flow excessive. So I set out checking the EGR (works fine w/ vacuum), cleaned out the EGR modulator, replaced all vacuum hoses I could get to w/ tight new ones, cleared it and went out again. No further 0402, but now it threw an 0171 (system too lean bank 1).

Now I'm thinking I may need to take the top off again and tighten up the intake manifold better (I swear, I torqued it to spec, but the car didn't run very smoothly until I tightened it and the plenum down pretty hard) as it may be sucking air somewhere (but still no 0135 or 0402! am I making progress?). I cleared the 0171 and went out again.

As I've come to expect, CEL popped on several miles later. Now I'm showing (get this...) 0130 (02 sensor circuit bank 1, sensor 1 (the one I replaced), 0133 (o2 sensor sircuit slow response bank 1 sensor 1), 0150 (02 sensor circuit bank 2 sensor 1), 0153 (o2 sensor slow repsonse bank 2 sensor 1) and last but not least, 0171 (system too lean bank 1). DEspite this hailstorm of codes, car runs very nicely.

I'd love to hear what some of the gurus think.


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So, I've thought a bit about this situation. Both o2 sensors prolly don't mean they are both bad, but perhaps just that they are reading way too much oxygen, top ending in the voltage output and not fluctuating as they should, creating code for malfunction in addition to the slow response codes bilaterally. A workable theory.

Took the top off again today, and started tightening down intake manifold bolts/nuts. They were quite easy to turn, but the specs says 13 lbs, and that doesn't seem quite enough. Slowly turned them down in sequence, so far haven't broken anything, but still not at a hard stop on any of them.

While I had the plenum/throttle body off, I took off the EGR and cleaned filter on the modulator and the passages to the valve. EGR works when vacuum applied. Am currently soaking the micky ficky screws on the idle air control in my home made brew of acetone and atf (50/50, works better than the very expensive penetrants I've read and seen) as I thought it'd be good to clean that as well.

Keep ya'll posted.


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The story continues. After putting it all back together after tightening up intake manifold bolts, it continues to throw the 0171, system lean bank1, sensor 1.

I decided to try an experiment, swapped the bank 1 and bank 2 o2 sensors. Now it's throwing 0130, 0133, 0150, 153, which means both sensors are showing malfunction circuits, both sensors are sluggish. What has remained consistant throughout is that the long and short term fuel trims for both sides are above 19.5+, and both o2 sensors stay down at 0.05 or below.

My theory now is that the original bank 1 o2 sensor is bad, but that there is a problem in bank 1, w/ high 02 read at the sensor.

Lean condition can cause this, and causes of lean conditions (I've read) are 1)faulty air induction (inspected w/out problems), 2) exhaust leak, 3) bad o2 sensor, 4) bad maf sensor, 5) clogged injector, 6) low fuel pressure, 7) bad pcv hose connection, 8)engine coolant temp sensor, 9) ECM.

W/ # 1 ruled out, I took the mass air flow sensor off and cleaned it, then checked to make sure appropriate voltage was getting to it, then checked the resistance on the sensor and that was ok too, so this is not likely the cause of the lean situation (#4 ruled out)

I think it unlikely that there is a clogged injector as the engine runs too smoothly. That makes # 5 unlikely.

Checked the pcv connection all looks good, new pcv valve, BUT I may take the hose off and ream it out, they tend to get pretty junked up (#7)

ECT sensor works well according to my obdII connection. (#8)

I've hear ECM's rarely go bad and when they do (corrosion on the board) the car won't run at all (#9)

That leaves me w/ #2, #3, and #6.

So....I'm thinking of taking the plenum and wiring harness off and trying to tighten down the driver side rear exhaust manifold bolts as I think I hear a little too much throaty exhaust sounds coming from that area. This would attend to #2. No cost except for the time. I've already tightend up the EGR connections and they look good on inspecition. I also tighted up the driver side exh manifold bolts (three) that are accessable from that side of the car.

Or.....I'll rent a fuel pressure guage and check that possibility out (while replacing the fuel filter, about $20), that'd would take care of #6. (never did this before but here's another learning opportunity!)

Or.....I could just go out and buy two new 02 sensors, but I'm too cheap. I'm waiting for my pick and pull to get some new es300 and or camrys in so I can try out replacement02 sensors, none available currently.

I'll keep ya posted, if anyone is interested.

Of course comments, advise, critical assessments of my logic are welcomed.


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More data today. Hooked up my computer to the car and took it out. Fuel trims were top ended at idle, but w/ acceleration/higher rpms would come down and even dip into the negative. O2 sensors would be very low at idle, but would come up and porpoise between 0.15 and 0.85 v w/ acceleration/higher rpms. Over all though seems way too lean, as the fuel trims should range from -10% to +10....and still got a 0171 code.

Cked the pcv pipe, no air leak there.

Ordered 2 02 sensors from Rock Auto ($104 for both including shipping), but still not sure they are the problem, have just been convinced (based on reading a lot about 02 sensors and fuel trims on line) that they should be replaced for best fuel effeciency, and longevity of cat.


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Well, it's not the o2 sensors. Got them by mail today, popped them in and there was no signficant change in the fuel trims. The sensors did bounce less sluggishly and will prolly be good for the future.

Now I'm left with trying to find out why I'm still getting an 0171 code, too lean bank 1. I'm now left to explore items 2 and 6 above, exhaust leak, low fuel pressure. And I'm still wondering about an intake leak as well.

I pulled off the plenum today -- with the goal of getting down behind bank 1 to try to tighten up the two lower passenger side exhaust manifold bolts which I suspect need to be tightened and may be the source of my exhaust gas leak. May get to them tomorrow.

I noticed there was some oil on the plenum intake gasket and can't figure out how it got there. I don't believe oil has any reason to being up in the intake manifold. The only place it could come from would be the cylinders and that would mean serious probs. But I pulled the plugs and they look very healthy, not losing/burning any oil. I am concerned that when I've installed the plenum (several x's now), that I have to leave the bolts and nuts to the intake manifold loose until I put the bolts for the rear stays into the back of the plenum. Maybe I'll just leave the stays not bolted and tighten down the plenum well. Also thinking of putting on some rtv on the gasket to make it a tighter seal.

I thought about trying to measure fuel pressure but you need a special toyota adaptor, to attach to the top of the fuel filter. Haynes suggest making one with a 12 mm bolt, but that looks like it needs a machine shop. May try to snag a banjo bolt and line from a junk yard...

I've not heard from anyone about this topic ... ignored? Or maybe there are no answers to the secret of life or 171 codes.

It does help to write about it as it organizes my thinking a bit.


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Today I took off the plenum and pulled off the wiring harness, then climbed on top of the rear head and was able to get a socket on the bottom two manifold bolts. They needed a bit of tightening. While I'd gotten down that far, I took off the timing belt cover and rechecked the timing marks -- they were perfect. Put it all back, but didn't attach the stays until after I'd torqued down the plenum, reusing the gasket. I'd thougth about using rtv but decided not to try that just yet.

I managed to break the spark plug coil connector on #6, so if anyone has one of these just lying around let me know. Napa doesn't carry them, so I'm gonna check w/ toyota tomorrow. I was able to connect the leads by hand and the car started ran fine.

But the fuel trim is still way off (system too lean), so I guess the exhaust leak etiology is unlikely now. I may take it into midas and have them "check" it out.

So my next step is to rtv down the plenum to eliminate the possibility that air is leaking in by that route. I think I'll replace the fuel filter and try to get a measure of fuel pressure somehow.

Questions now: 1) could there be an injector that's not working well even though the engine runs smooth?

2) does anyone have a spark plug coil connector?

3) has anyone run w/ a system too lean CEL long term? cause any problems?


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  • 1 month later...

Interesting story...

New poster here, just wishing you good luck.

I have a 2000 ES300 that I just got done fixing P1130, 1135, 1150 and 1155 codes for. I had to replace both A/F sensors. I take it you hecked the resistance on the old ones and the new ones before you installed them.

During my research, I did hear that you are better off getting the OEM ones from DENSO direct from Toyota/Lexus dealerships. Aftermarkets tend to not play nice. I found the $213 ea. a bit hard to swallow and got DENSO aftermarkets at NAPA for $154 ea. I found your $104 price you quote pretty exceptional so I am just wondering if those sensors aren't the right ones or maybe just bad from the start.

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Thanks for the feedback, your like the only one who has responded!

I did think about the aftermarket o2 sensors, but they seem to porpoise nicely, and I'm convinced that they're all made in China anyhow. I've used similarly inexpensive ones on some of my other cars with good results. The "belief" that one should only use lexus/toyota parts is a bunch of hogwash desiged to enrich further one of the richest car companies in the world. Furthermore, I think it is highly improbable that 1) both sensors wouldn't be equally good/bad (rear bank only code), and 2)the fuel trim action on my computer would remain exactly as the replaced (old) sensors.

I haven't done much since my last post on this problem, but since the car is running quite nicely, I'm thinking I might just live with the system lean code (black electrical tape over the light?) I read somewhere this may even enhance my mpgeez.

I've shifted my focus to the fact that I have no overdrive (except about 2% of the time), desipte no dash light to indicate such a malfunction (od off light comes on when I hit the button...)

I took it to a trans shop and they felt it was electrical. I took it to the elec guy who thought there was an 80% likelihood its a bad ECM. He didn't want to replace it as he didn't want to eat the cost of a new one in case that wasn't the problem. Neither shop charged me for this diagnostic work :)

So now I'm looking for an ECM, thinking a used one vrs refurb. Decided to wait to see if one came up on ebay or at the local picker yard for a reasonable price. Need an 899661-33340 incase anyone has one ('94). Turns out there were several ECM's for that year. I'm kicking myself as last fall I had my hands on the exact one at the upullit, woulda cost me $30 but I didn't think I had a need for one at that time. That es was crushed after Xmas (along with the ECM). Hindsight is 20/20!

I'm thinking that my 0171 code might be a manifestation of a bad computer as well. Number 9 in the list way up above. Time and a replacement ECM will tell.


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We have found on both lexus sites that aftermarket air/fuel sensors dont like these engines...You can get them on Amazon for around $100 apiece (Denso oe MANUFACTURER) I remember that was the fix for one guy fighting the codes that seemed to keep coming back. As for the plenum, I had to replace my gaskets because of a water leak over by the thermostat. Torqued the new gasket to spec and never had to touch it again. As for air leaks dont forget the flex pipe on the exhaust which may be leaking air. Most of the time it just makes noise...I have read of like no ECM's being bad, but I suppose it can happen. Good luck keep us posted. By the way the fuel filter is in the gas tank, and nobody has ever posted it being a fix as it is a mesh screen that is self old RX300 had the original with 188k miles...

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I suggest we agree to disagree about the o2 sensors. I try to work logically, not just replace things willy nilly. I also don't just "believe" that denso is better than any other. I always like to have some real proof that it is worth spending twice as much for a given product before doing so.

Regarding the plenum, you mentioned replacing the gaskets (plural). Do you mean the intake gaskets? I've thought about doing that as well. As for the fuel filter, it is up front for this version of the 1mzfe. Easier to replace out in the open. I have a Calif version of the 94 es, and it has two "flex" pipes (they're actually the most unflexible pipes I've ever tried to work with...), both of which have been tightened down as much as safely possible, and visual inspection from underneath w/ mirror shows good contact/mating. No current exhaust noise.

And I agree, have never heard of ECM going bad, but the elec guy did indicate this happens, more often than one would expect for a solid state device.

But what do you think about just running with a 0171, will it do any harm?


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