1997 1.5, engine coolant temperature sensor (getting a catalytic converter CEL)

[brant914]

I'm probably going to end up buying a new upstream cat for my 1997 protege 1.5 manual.
the car has 120K on it, and I've only owned it about 5k
I've done LOTS of service on it, eliminated 3 or 4 check engine lights, new IAC, cleaned EGR, timing belt, water pump, plugs and wires, cap-rotor, replaced the distributor due to a coil problem, both O2 sensors new, etc......

my guess is that my intermittent ignition mis-fire probably damaged the catalytic converter
but before I buy a new expensive cat, I wanted to rule out everything that could possibly cause the 420CEL

so I've tripple checked my timing
I've had witch-hunter rebuild my injectors
and I'm reading other possible causes of a P0420 are:

1) exhaust leak in manifold (wouldn't I hear this... I don't seen or hear anything but is there a better way to check?)
2) down stream o2 not working (I have replaced this with new)

3) engine coolant temperature sensor not working properly

so here is my real question
is there a seperate coolant temperature sensor for the computer and for the dash water temp gauge?
I have replaced the thermostat and my dash gauge is functioning normal
does the computer use a different coolant temp input that could be my culprit?


thanks in advance anyone
brant

 

[concept]

Are you sure that the coolant sensor is a possible cause? Do you not have access to a service manual? My guess would be that the cat needs to be replaced ($139 at rockauto).

 

[brant914]

the coolant sensor failure is one of the possible causes in the service manuals

I've heard that the cheap cats are a waste of money and won't last

so I just want to verify everything before purchase

bump...
anyone know if the gauge and ECM use the same coolant sensor or seperate ones?

brant

 

[brant914]

it looks like there might be 2 sensors in the haynes pictures...
of course you never know what year haynes is referencing so we will see tomorrow in the garage.

 

[concept]

I don't think Walker cats are cheap. See:

http://www.walkerexhaust.com/products/catalytic-converters/epa-compliant

However, if you are planning to keep your car for another 20 years, there are more expensive cats out there.

 

[brant914]

I agree... I've heard good things about the walker cats
I don't think they are cheap either.

the first or primary catalytic converter sold by rock auto runs $181
the walker version runs $250 for a primary unit.

and its the primary unit that is measured with the 2 oxygen sensors and throws the P0420 codes

so it was the "davico" ? brand converter at $181 I was referring to as cheap

 

[brant914]

Ok... there is only one coolant temp sensor on a 1997
I unplugged it and ran the test cold
(its a little on the low side cold)

but then ran the car for 15 minutes in the garage
the water temp gauge on the dash was pegged at cold.
the sensor numbers came down to within the range they should of for warm

I plugged the sensor back into its loom and suddenly the gauge on the dash is in the low operating region

 

[brant914]

just for kicks I pulled the exhaust manifold shield and looked everything over again
no visible cracks and more importantly, no sign of heat marks or carbon marks..
I am 99% sure there are no exhaust cracks at this point.

I don't have a scope to verify the O2's again... but I did have a shop run that for me last month.

looks like I'm saving for a walker primary cat now.

 

[concept]

I plugged the sensor back into its loom and suddenly the gauge on the dash is in the low operating region

Do you mean the gauge was indicating that the coolant was still cold?

 

[brant914]

Do you mean the gauge was indicating that the coolant was still cold?

with the fuel injection ECM coolant sensor unplugged the dash gauge was dead
I ran the car for 12-ish minutes in the garage with no real load
the HVAC system was just starting to warm up at the vent, so I knew the engine coolant temp was starting to come up, but probably not full operating temp yet

I reconnected the coolant temp sensor and the dash gauge jumped from absolute zero to the lower end of the normal operating temp scale
I didn't expect full operating temp as the car had only been running for about 12 minutes without load

so I confirmed that the one sensor does operate both the ECM and the dash gauge
and I've already confirmed my thermostat and dash gauge do get up to full operating temp