I'm looking at a 98 with 156,000. Among other minor issues, it has p0171 and p1133 codes. Looks like the sensor in the exhaust manifold has been changed recently. Also, the exhaust, just behind the flex area, before the cat, has broken off, giving a large, noisy, leak.

Could this leak be a cause? if not, what else?

 

The exhaust leak can cause the P0171 code.

P1133 code is specific to the O2 sensor. Bank 1 sensor 1. This is first sensor on the exhaust system when coming from the cylinder head.

P0171 can also be caused by a bad 02 sensor.

If a new sensor was installed try to determine the brand and type. Bosch brand and universal type sensors (one sensor wiring fits many car models with some modification) can have issues in Toyota. Denso OEM (the sensor wiring fits your car model out of the box) and Toyota sensors work best.

Make sure the sensor is the correct part number to fit your engine.

The 4 cylinder was certified to either California or Federal emission standards and the sensor for a California car may not be the same as a Federal car. People have installed the wrong sensor on more then one occasion. There is a sticker under the hood stating if the car meets CALIF or FEDERAL standards.

If buying the car include in your price offer having to fix the exhaust and replace the sensor. Assume the sensor has a problem unless the seller is willing to have it checked by a shop.

The sensor is easy to replaced, a DIY job. RockAuto Auto Parts has the lowest on line prices. Call a Toyota dealer for their prices which will be higher. You will need to know if the car meets Federal or California emissions standards when buying the part.