02 engine bucking-lost power-4 codes
 

just had transmission rebuilt-timing belt replaced-new plugs.ran good for week or 2 now loosing power & at stops engine dies. obd codes were initially 121/171/304.got it home somehow & now just shows 171/304. not sure what to do.valve cover gasket also replaced including seals for the new plugs. any suggestions. car only has 69k miles.

P0121 is a throttle position sensor code
P0171 is a lean code
P0304 is a misfire in cylinder 4

P0121 seems to have cleared itself up for now

P0171 and p0304 sounds like a fuel issue.
Could be
1. Clogged injectors
2. Clogged fuel line
3. Faulty fuel regulator
4. Faulty fuel pump
5. Faulty associated wiring of fuel system

I have seen bad grounds cause spark to not be as good as it should be
And have seen bad ground on a single injector make bad misfire
Clogged injectors or bad fuel pump??
Fuel regulator not so much

Well, would like to see how you go about fixing this
Please keep us updated

Note: Power loss can also be caused by VSS and ABS speed sensors not giving good info to ECU
ECU will put engine into limp home mode
But, OBDII scanner would give fault code for this.

Thanks for your suggestions. This has been a real puzzle but finally resolved. It became so bad the car had to be towed. My mechanic was puzzled too but thanks to year's of experience determined it might just be the MAP sensor which it was. Why it never flagged the error code is a mystery. I've read elsewhere that a good test is disconnecting it while running & if the engine suddenly quits the map sensor is good & if no change its bad. Thanks again for your helpful suggestions.

Thanks for the update.
I have seen MAF sensors do this but didn't know a MAP sensor would also
Thanks again

Thanks for your suggestions.