No "Check Engine Light" with key on engine off.

[gary49]

I have a 1998 GEO Tracker 2wd 1. 6L I4. I went thru local emissions and failed. The main reason being that there is “NO” “Check Engine” light with the key on engine off. I’ve only had the car for a short time and have never noticed a check engine light. It also showed P0135 ( 02 sensor heater circuit malfunction-Bank 1) P0340 (Camshaft Position Sensor Circuit Malfunction) P0500 (Vehicle Speed Sensor Malfunction) . I recently had a oil leak and the battery was unhooked for this procedure. My questions would be: Is there a particular fuse path I would search out that would cause there to be no “Engine Light” and is it possible that the same wiring problem has caused all the problems. THE CAR RUNS GREAT !! It starts good and gets to normal operating temp just fine and drives down the road just like everything is normal. I checked the fuse box under the hood to the best of my ability and the fuses appeared to be good. There was no fuse in the ABS slot but when I put a good one into the slot it didn’t make a difference so I am guessing that it doesn’t need that fuse. Also, where on the instrument cluster would I see the check engine light if it were lit… Perhaps this will give me a heads up when I tear the dash apart …..LOL……Any Suggestions….Thanks…. basgb@charter.net

[Rhinoman]

You might be able to see the CEL position if you shine a torch into the dash. The previous owner may have removed the bulb to mask the problems. The O2 sensor heater will be on a different circuit to the VSS and CPS.

[jtgh]

that fuse is needed.
your PO. is evil.

put the lamp back in.
fix O2 (its bad.)

if you want schematics , hollar.

[gary49]

Is the 02 sensor the one before the CAT CON.. (bank 1 sensor 1)..& will a bad 02 sensor cause small surges in the cars performance till it reaches normal operating temp.......

[CodyJ]

Time to time, because it does not know what the AFR mixture if it is bad.

You could try resetting the codes after replacing the ECU and seeing what you have. Local auto parts stores will do this for free, just ask.

[ranger88den]

Originally Posted by gary49

Is the 02 sensor the one before the CAT CON.. (bank 1 sensor 1)..& will a bad 02 sensor cause small surges in the cars performance till it reaches normal operating temp.......

Yes, it's threaded into the exhaust manifold. I never had the surging problem on my 97 when the O2 was bad, it may be related to the other codes. My understanding is, always fix the lowest numerical code first then work up, so repair/replace the O2 sensor and see what happens. Good luck

[jtgh]

the o2 has MANY failure modes. each with there own set of symptoms.
open heater. ( car fails to enter closed loop, wastes gas, runs rich, rich while idling !)
shortred heater (can force car engine to lean side. & bogging shorts to the cell side are bad,very bad)
dead o2 cell. poor gas mileage and other drive-ability issues. (mAy go rich )
slow 02 (OVER 100K miles), just poor gas mileage.
stuck low, or stuck high, CELL, can cause very bad running problems.
EACH SETS A CODE.
CELL = O2 SENSOR CELL (acts as a small 1 v battery)
Heater, it is just a heater, it heats up and keeps the O2 running hot at all times.


it also depends on if something else is bad. like the car is already running lean, and the O2 signals rich, so the ECU signals (stupidly) double lean. a BOG monster is created.

most codes below 400 are catastrophic fails.
there are Catastrophic failse above 400. (eg P0444 and sim)
OBD-II Check Engine Light Trouble Codes
the FSM tells you which to trouble shoot first.

if you get a group of codes. fix the stuck at codes first. hard fails first. (opens, stuck high for example)

for example.
always fix , sensors that are grossly in error. (STUCK LOW, STUCK HIGH)
heater on o2 fail is a gross fail. it says, the heater is OPEN circuit. no ifs, ands or butTs.
TPS open (STUCK HI)
TPS shorted. (STUCK LOW)
are all dead TPS.

The test monitors all have a parametric page , explaining what things must be good before the one you are looking at , can be trusted.

a classic case in point is EGR fail.
Never trouble shoot EGR P0400 fail (or 51) until all the other errors and bad sensors are
corrected. (but I do make sure valve closes fully first)
listed under Preconditions.

The more complex problems. that can be fooled are
EGR fail.
MAF low ( illegal air leaks anyone)
VSS fail ( it thinks you are driving when you are not)
or O2 failure to show lean during Deceleration test monitor ( car running rich can set this DTC as a false Positive)
TPS failing to show idle, ( this can be false if MAF or MAP are sending false data)
P0420 or similar LAZY catcon fails are complex.
TPS idle fail.
The ecu , looks at rpm and sees low, and MAP high (or MAF low) and thinks (sick) the
car should be idling, but is not, because the idle switch is open)
this can trip falsely if the MAP is reading too (hi vac falsely) or the MAF is reading low falsely (and is actually high high air flow).
There are 2 types of test, DIRECT (EGR HEATER dead) is one, and the indirect type.
"In directs" tests,(monitors)are very complex and are easy to fool.
(bad sensors, or bad engine)

not to mention air leaks to the induction sets both sensors to false regions.
or a stuck intake valve, or valve lash set less than spec. (less then 0 holding valve open)


the key is to look at what are the failure rules. ( how is this test ,conducted ?)
if it says it is looking at 3 sensors, then it might be them wrong and not the DTC it determines to be wrong. (GIGO , garbage in results in garbage out, always true on computers)

It can be hard to get a print out of the Monitors, but it is found in the FSM.
GM also publishes this data.
So does SAE.

if you have a specifice DTC code and want the full truth of how it works, just post here.
we can dig them all up..

cheers. and happy trails.