If chapter twelve was about Oliver's potency, chapter thirteen was about Clifford's impotence, manifested in a broken bath-chair.
Clifford decides to take a break from industry and take a walk in the woods with Connie--her walking, him sputtering around in the 1920's equivalent of a power scooter. This is a chair designed for level, paved English gardens, not the hills and undergrowth of the forest. There's a path smoothed out by use, so his chair does alright until they come to the crest of a hill, the slope of which dips and then rises again to the well near Oliver's cottage. When they leave, the chair makes it down the small slope from the well and into the valley between the two slopes. Then: "It was a steep and jolty climb. The chair plugged slowly, in a struggling unwilling fashion. Still, she nosed her way up unevenly, ... then she balked, struggled, jerked a little way out of the flowers, then stopped" (204-205).
Clifford's chair stalls. He decides to "let it breathe" for awhile, so they put a stone behind the wheels and wait. When Clifford starts the chair again, "it struggled and faltered like a sick thing." Connie offers to help the poor motor by pushing the chair, but Clifford will hear none of it: "What's the good of the damned thing, if it has to be pushed?" Another pause to let it breathe, then another start, "more ineffectual than the first." (205) Connie keeps begging him to let her push, or to sound his horn for Oliver to come and push.
The chair continues to fail, and Clifford becomes angrier. Oliver says that a push will help the chair along, but Clifford is adamant that the chair can manage on its own. He fights with its levers and gears, and the chair lurches sideways: "But the keeper had got the chair by the rail. Clifford, however, putting on all his pressure, managed to steer into the riding, and with a strange noise the chair was fighting the hill. Mellors pushed steadily behind, and up she went" (207). Clifford cries out victorious, until he realizes that Oliver is pushing, at which point he snarls at the keeper to leave off.
"The keeper stood back [...] The chair seemed to strangle immediately. She stood inert. Clifford, seated a prisoner, was white with vexation. He jerked at the levers with his hand, his feet were no good. He got queer noises out of her. In savage impatience he moved little handles and got more noises out of her. But she would not budge. No, she would not budge. He stopped the engine and sat rigid with anger" (207-208).
Oliver pokes at the engine, Clifford tries her again, and they stutter forward, "the engine doing about half the work, [Oliver] the rest." Clifford snaps at Oliver again to stop pushing, and his chair rolls backwards. Again, he's saved by Oliver catching his chair, to which he gratefully replies: "It's obvious I'm at everybody's mercy!" Finally, he orders Oliver to push him home; but the chair's brake jams, and it won't move. To fix it, Oliver lifts the back end of the chair, Clifford and all, and orders Connie to tug at the brake. Connie begs him not to, for two reasons: the chair is heavy (look at the picture again), and Oliver is weakened by pneumonia he had in the past. She's worried that he'll hurt himself; but they get the brake fixed.
Oliver proceeds then to try and push the chair up the hill; fearful of the keeper's failing strength, Connie pushes too, and they have a lovely little moment right behind Clifford's head: "She looked at his smallish, short, alive hand, browned by the weather. It was the hand that caressed her. She had never even looked at it before. [...] All her soul suddenly swept towards him [...] Shoving with his left hand, he laid his right on her round white wrist, softly enfolding her wrist, with caress. And the flame of strength went down his back and his loins, reviving him. And she bent suddenly and kissed his hand" (210).
Clifford's chair is personified as female, like cars and ships, and this scene with the breaking chair may be read as a metaphor for Clifford's sexual impotence. Let's look again at the quote from pages 207-208: "He jerked at the levers with his hand, his feet were no good. He got queer noises out of her. In savage impatience he moved little handles and got more noises out of her. But she would not budge. No, she would not budge." If we imagine the mechanism of female genitalia in place of the levers and handles of his chair, Clifford sounds a lot like a man who can't give the woman an orgasm. He is impotent, in the sense that he has no strength, stamina or ability in the bedroom for himself (the traditional definition of impotent: unable to sustain an erection) or for the woman he is with--unlike Oliver, who is all potent stimulation and sensation for both himself and Connie.
Speaking of Oliver and Connie: "It was curious, but this bit of work together had brought them much closer than they had been before" (211) Toiling together to push the chair up the hill is representative of other struggles they may face together, and in this they lent strength to each other. It's interesting to imagine if this would have happened for Clifford and Connie, if he'd only let her push the chair in the beginning.