When its very cold outside my 2003 corolla will not start just changed plugs and temp senser we can put a heat light at the firewall by the temp senser for 30 min or so and she will usally fire.

Wow, no engine option stated, !!!

how cold is cold, we dont know, ALASKA -50F or Keywest at +80F

what temp sensors , there can be many.

my 08 has 3 on engine,. one in tranny, and 1 in front corner for my outside airtemp option with compass.



the one that counts is the ECT, and this sensos is screwed deep

into the waterjacket or in the thermostat housing, vastly hard to heat up by hand that. for sure heating up the thermal mass of and engine

with a heat lamp is near impossible. so where is it. EXACTLY.



are you running old fuel.?

running summer fuel?

Is CEL (MIL) lamp glowing when it does run?????





hard cold starts are caused by many things.

Just too cold is one (spend time in alaska and see)

Weak spark, check it at each

you connect scan tool, early in the morn. and see if the ECT and IAT

read near the same and near outdoor (where car is parked) temperature. if yes, he ECT and IAT are working.

once started the ECT will rise to 180F or more and hOLD; does it?

if in fact you did by change warm the ECT artifically and start easy

that means the engine is flooding.

my guess is you didnt do diagnosis at bad start time.

(spark tips soaking with fuel) leaking injectors on one or more cyl?

check all sparks for flooding, not just one,

if it is flooded, the operators guide says (RTM) top crank with the throttle held open and this cuts fuel and flood engine starts.

other causes aplenty

too high fuel pressure

leaking injectors

weak spark at any cylinder, got P030x DTC codes from scan tool?

weak compression on any cyl. (sticking valves? cold) (gage it)

if the engine is good, good compression that leaves 2 more.

weak spark (if all spark plugs are strong cranking) then....

or bad fueling,



bad fueling can be.

bad fuel

leaking injectors, (for 3 reasons)

ECU is overfueling, (ECT reads way too low than reality)

The engine has 2 more sensors that can cause over or under fueling

THE MAP or MAF, you didnt say which engine so cant guess.



MAP vacuum port clogged with PCV cloud of death , gunk,.

MAF hot wire coked up, clean it.

the maf coked will cause lean starts.

the mAP can fail 2 ways, stuck at high or low pressure.

stuck high is way more common, this causes flooding.

there are other reasons.



1: EGR main stuck open ( bad news. it floods the intake with exhaust gases and that really wrecks combution to start any engine.

2: ISC stuck closed, (flooding)



or a MAF based engine with any form of induction leaks (vacuum leaks)



i think i hit them all (hope), and there are tests for each case.

ask.