98 jeep codes p0340+po304+po138+p1391 where should I start to fix

SOURCE: 2001 Jeep Grand Cherokee code P1391

I would check to be sure that the timing chain hasn't jumped at all ..

SOURCE: have 2000 Jeep Cherokee with P0340 code. Cranks

crankshaft position sensor. verify 5v reference voltage..... where and how to check it depends on what engine. its really important to tell us ALL the facts. Cherokee or Grand Cherokee, 4 or 6 or 8 cyl. what have you checked, and the results of what you have checked.

SOURCE: 2006 jeep liberty code po304

P0304 indicates misfire in the cylinders..
A misfire can be triggered by several situations. inadequate fuel, ignition misfire, insufficient air flow, too much fuel, incomplete compression... If you look at freeze frame data with a scanner you can see what the conditions were when the code was set, this will really help to pinpoint the problem.

SOURCE: codes p0351, p0352, p0353 and

obdii-code-p0351-x2-704058/index4.html">http://www.jeepforum.com/forum/f13/help-obdii-code-p0351-x2-704058/index4.html

This thread basically says replace the crank position sensor and the cam position sensor. Check the wiring for the Oxy sensor(s). And if all else fails then change the PCM.

SOURCE: 98 jeep grandcherokee code PO138,

P0138 - O2 Sensor Circuit High Voltage Bank 1 Sensor 2

The rear heated oxygen sensor (or sensor 2), after three way catalyst (manifold), monitors the oxygen level in the exhaust gas on each bank. Even if switching characteristics of the heated oxygen sensor 1 are shifted, the air-fuel ratio is controlled to stoichiometric, by the signal from the heated oxygen sensor 2. This sensor is made of ceramic zirconia. The zirconia generates voltage from approximately 1V in richer conditions to 0V in leaner conditions. Under normal conditions the heated oxygen sensor 2 is not used for engine control operation.

Causes:
- Harness or connectors (The sensor circuit is open or shorted)
- Rear heated oxygen sensor (Bank 1) may be faulty

Solutions:
Replacing the O2 Sensor 2 usually takes care of the problem


P0141 - O2 Sensor Heater Circuit Malfunction Bank 1 Sensor 2

Causes:
- Harness or connectors (The sensor circuit is open or shorted)
- Rear heated oxygen sensor (Bank 1) may be faulty

Solutions:
Replacing the O2 Sensor 2 usually takes care of the problem


P0463 - Fuel Level Sensor Circuit High Input

The fuel level sensor is mounted in the fuel level sensor unit. The sensor detects a fuel level in the fuel tank and transmits a signal to the ECM or PCM, It consists of two parts, one is mechanical float and the other side is variable resistor. Fuel level sensor output voltage changes depending on the movement of the fuel mechanical float.

Causes:
- Fuel level sensor circuit harness or connectors is open or shorted
- Fuel level sensor may be faulty

Solutions:
Replaced fuel level sensor.